Echopype

New features
- Add EK80 support:
- File conversion from EK80 `.raw` files to netCDF and zarr formats
- "Simple" calibration to frequency-average Sv based on pulse compression output is implemented but needs to be thoroughly tested.
- Rename subpackage `echopype.model` to `echopype.process`
- The new name better describes the subpackage's function to process data for further analysis
- Also rename class `EchoData` to `Process` to mirror the structure in `Convert` better.
- Importing using the old names will be deprecated in the next release.
- Overhaul converting multiple files with `combine_opt=True`
- If target format is netCDF, temporary files will be created and finally combined to a single netCDF. This is due to current restriction that xarray does not allow simply appending new data to an existing file.
- If target format is zarr, data in each file are unpacked and appended to the same output file.
- Allow reading Zarr into `Process` in addition to netCDF: thanks lsetiawan!
- Add a logo!

Bug fixes
- Fix bugs in slicing NMEA group data based on the same time base when `range_bin` is changed

This minor release includes the following:
- fix the bug in remove_noise function for AZFP data (issue 110)
- update testing to use both python 3.7 and 3.8
- include how to pull test files using git LFS in documentation
- remove a stalled command line file converter that won't be further developed

This minor release includes a few bug fixes:
- fix zarr saving error in the Provenance group in the previous release
- fix bugs in updating environmental parameters (used for calibration) for EK60 and AZFP
- allow users to update calibration parameters in EK60
- update notebook EK60_demo_OOI.ipynb to use new raw data archive folder structure

This release contains updates to the convert and model module:
- Correct file parsing and storage for split-beam angle.
- Allow user-defined environmental variables to be used in calibration.
- Accommodate EK60 setting variabilities in the `range_bin` dimension.
- The raw file will be split into multiple .nc files if the length of `range_bin` changes in the middle.
- For cases when different frequency has different length of `range_bin`, the shorter channels are padded with `NaN`.
- Allow combining multiple files during unpacking.
- Options to:
- overwrite previously converted files.
- specify output directory and filename postfix for `.calibration()` and `.get_MVBS()` operations.
- Add support to convert to .zarr file. Note processing functionalities have not been fully supported.

This release includes major changes to the package structure and functionality improvements:
- Added support for handling `.01A` data files from the AZFP echosounder.
- Changed the module structure to enable a uniform interface to work with EK60 and AZFP data.
- Improvements to EK60 components:
- Included NMEA datagram unpacking in file conversion routine.
- Added handling for pings with different range setting within the same data file. This feature is experimental and the default behavior will change in future release.

Minor update:
- use versioneer for auto-versioning