Fstd2nc

* Re-enabled unlimited time dimension for applicable files.
* Renamed zaxis,zeta,eta,sigma axes to just 'level'.
* Removed old anciliary_variables stuff for A/B vertical coeficients.
* Added formula_terms for vertical coordinates, along with a "pref"
(reference pressure) scalar variable.
* Loosened criteria for matching vertical (!!) record when there's only
one vertical record in the file.
* Fixed bug when there are multiple reftimes in a file (issue 15).
* Added support for time-dependant lat/lon coordinates from LA/LO
records.
* New --exclude option to remove variables and degenerate axes from
the netCDF file.
* New --profile-momentum-vars and --profile-thermodynamic-vars options
to specify which variables are on which type of level.
Note: only needed for profile data.
* New --missing-bottom-profile-level option to work around a bug in
profile data coming from rpnphy output.
* Fixed a bug in the times for profile / series data.

* New --rpnstd-metadata-list to select a subset of RPN metadata
attributes to copy to the netCDF file.
* Made --minimal-metadata the default. The old behaviour (copying all
RPN metadata) can be triggered with a new --rpnstd-metadata option.
* Coordinate-related RPN metadata is no longer copied, since the values
are already encoded in the coordinate variable.
* Restored support for 'A', 'B', and 'G' grids.
* Fixed the repeated longitude in global grids - value is now 360
instead of wrapping back to 0.
* Use consistent lat/lon/station coordinates for series data when using
a --filter on the ip3 values.
* Unused axes are now pruned from the file. Mostly affected series
files, which had an extra time axis from the unused datev values.
* Vertical axes are now encoded as floats, since that's the precision
of the original ip1 values. The pandas version of the iterator was
upconverting these to doubles, which would cause different results
depending on whether pandas was installed or not.

* Improvements to xarray interface to utilize the CF conventions
metadata (such as auxiliary coordinates).
* Add station, leadtime, and reftime as auxiliary coordinates in series
files.
* Use the date of validity by default. Old behaviour (date of origin)
can be restored by using --dateo or --forecast-axis options.
* Added 'leadtime' and 'reftime' auxiliary coordinates to encode the
forecast information.
* Progress bar is enabled by default. Can be turned off with
--no-progress option.
* Added CF metadata for various grid projections (rotated pole, polar
stereographic, lat-lon).
* Decode surface types (ice, land, sea, etc.).
* Decode depth coordinate for ocean model output.
* Fix decoding of masked data in raw binary (X32) format.

* Fixed unlimited dimension specification for PyDAP interface.
* Workaround for bug in numpy 1.14 when using masked structured arrays.
* Fix forecast times for series files.
* Don't discard LA/LO fields. They needed in some circumstances such
as trajectory files.

* Faster scanning of input file headers.
* New --progress option to show a progress bar for long conversions.
* More speed optimizations for RPN date and IP1 value conversions.
* Faster determination of dimensions from the record headers, using
pandas package.
* Restructured the package so each feature is in a separate Python
module, for easier development.
* Added xarray interface to facilitate custom netCDF conversions from
Python.

* Faster I/O when converting multiple input files.