Readgssi

- added support for the 5103 400 MHz antenna, and an antenna called `CUSTOM` in the header, for which center frequency is not known but is likely adjustable
- added support for the 350 HS hyperstacking antenna
- fixed a bug that created an extra matplotlib axis
- added rounding to epsilon_r value on y-axis depth-type display
- added a patch for [12](https://github.com/iannesbitt/readgssi/issues/12) to improve axis sizing and tight_layout() handling
- changed background removal (BGR) function to operate similar to RADAN boxcar function
- this change means that you can no longer simply specify `-r` for full-width background removal. new nomenclature for full-width BGR flag is `-r 0`. `-r 100` will apply a 100-trace rolling boxcar BGR (after normalization and stacking)
- this is done with the `scipy.ndimage.filters.uniform_filter1d()` function and means that `scipy` is now explicitly required
- function will automatically filter out low-frequency washout noise caused by bgr, using obspy's implementation of scipy's butterworth bandpass from `0.1*f` to `10*f`, where `f` is antenna frequency. nyquist is automatically accounted for by obspy.
- BGR value is displayed in title if set > 0, or as "full" if BGR is full-width (i.e. `-r 0`)
- added zerophase vertical triangular FIR filter, and changed default filter from butterworth to triangular to attempt to match that in RADAN. butterworth is no longer used (but kept for posterity).
- this function uses the `scipy.signal.firwin()` filter design and `scipy.signal.lfilter()` implementation
- added ability to suppress plot title using `-T` or `--titleoff` from the command line or title=False in main python function
- changed `ns_per_zsample` in header to read data shape from header rather than array size
- added a bug fix for filtering that caused the sample rate to be set too high
- significantly improved triangular bandpass filter, so that it nearly matches RADAN
- when Z axis value is set to "samples", the zeroed samples are taken into account. so, if the array had 150 samples sliced off, the plot axis will now start at 150 instead of 0
- added per-channel handling of timezero. you can pass a list with `-Z 233,150,92` (or `zero=[233,150,92,None]` in python). defaults to `-Z 2` (`zero=[2,None,None,None]`)
- added programmable zoom with `-e left,right,up,down` flag and list
- cleaned up some dangling floats
- updated help text and README help text
- added automatic tight bbox axes for savefig
- changed from 150 to 300 dpi for easier poster printing measurements
- fixed calculation for two-way time (previously per [17](https://github.com/iannesbitt/readgssi/issues/17), calculation erroneously displayed one-way time)

- updated examples in README
- fixed [16](https://github.com/iannesbitt/readgssi/issues/16) which incorrectly applied GPS array values for normalization

- fixed [13](https://github.com/iannesbitt/readgssi/issues/13) by adding `SS MINI` to list of recognized antenna codes
- fixed bug ([15](https://github.com/iannesbitt/readgssi/issues/15)) which caused runaway memory usage when distance-normalizing large files. files are now distance normalized in chunks.
- fixed a bug that caused a read failure on DZG files with RMC strings before GGA (iat function didn't like my slicing)

- fixed bug that prevented reading NMEA RMC-only DZG files. now fully compatible with DZGs created with [gpx2dzg](https://github.com/iannesbitt/gpx2dzg)
- added `-d` flag documentation to help text and readme
- updated docstring formatting
- added PyPI downloads badge
- added ability to manually set samples per meter using `-d <float>` flag
- added support and badge for travis-ci in anticipation of test suite
- `-V` or `--version` outputs version string
- fixed bug ([10](https://github.com/iannesbitt/readgssi/issues/10)) which read NMEA GGA strings incorrectly from DZGs that don't contain NMEA RMC
- fixed bug ([11](https://github.com/iannesbitt/readgssi/issues/11)) which prevented SIR-20 files with antenna code `GSSI` from being read correctly

- addresses [7](https://github.com/iannesbitt/readgssi/issues/7) to add support for DZT files created with gprMax
- added explicit support several antenna codes specific to SIR-3000 units and before
- added the ability to label radargram axes with distance and time units
- created readthedocs support files in anticipation of more full documentation and a future stable version release

- a patch was added that includes several important bug fixes
- added support for the `400MHz` antenna type
- fixed a bug that affected the stacking routine
- fixed a bug with the time zero variable not being available when implementing the naming scheme
- fixed a bug with plotting that resulted from a key error in the header variable
- moved dzt reading-related functions to `dzt.py`
- changed tab spacing aesthetics in two plotting routines
- fixed a bug that caused errors reading multi-channel files
- script has more degrees of freedom while handling unknown antenna types
- script should handle unknown antennas gracefully
- script will try to extract frequency information from antenna name (harder than it sounds given GSSI naming inconsistency)
- added ability to reverse radargram (i.e. flip horizontal left to right)
- fixed bug with command line autostack
- changed command line verbose messages
- added ability to distance-normalize profile using GPS records
- currently can read .DZG GPS records only, but plan to add ability to read .csv (lat, long, time) and NMEA GGA/RMC textfiles
- changed naming scheme to be "more readable"
- changed filename differentiator from frequency to channel number (makes sense because theoretically, a user can have the same frequency antenna on two different channels, which would result in a naming conflict)
- fixed a bug in normalization caused by an errant format string
- stacking algorithm now uses reduce function to create copy array
- normalization will now automatically set traces per meter value of `header['rhf_spm']`
- added export numpy binary file (.npy)
- added header export to json
- added export to gprpy (.npy & .json) per [this discussion](https://github.com/NSGeophysics/GPRPy/issues/3)