Fstd2nc

* Added north_pole_grid_longitude attribute for rotated grids.
* Fixed missing vertical coordinates in series data.
* Fixed bug when using --forecast-axis with series data.
* Fixed bug with --yin and --yang when there are non-YY fields in the
file.
* Improved precision of lat/lon axes for non-rotated 'Z' grids.
* Switch to default of fused=False in .to_xarray(), since fusing the
records can give a performance hit for certain access patterns.
* Added $CMCCONST to search path for --opdict.
* Check if file specified by --metadata-file actually exists, instead
of ignoring the missing file.
* Fixed get_crs() for data generated with no_adjust_rlon=True.
* Register this as a backend entrypoint for xarray, so now it should
work from xarray.open_dataset().
* Fixed interpolation for vector fields like UU, VV.
* Added support for RPN extended date range (for years 0-9999).
* Registered a .to_fstd() method for xarray.Dataset objects, for some
preliminary testing.
* Fixed an array transpose happening in some conditions for
.to_fstpy() and .from_fstpy().
* Debian packaging rules provided by Tom Kralidis.

* From Python interface, allow interp parameter to accept existing
grid ids as input.
* Fixed a crash when converting masked data when some of the mask
records are missing.

* Fixed crop_to_smallest_grid option, which was broken in .to_xarray()
after fused records were implemented.
* Added doc string in .from_fstpy().
* Improved support for mesh grids (grtyp='M'). Removed extra
dimensions from mesh_face_nodes variable that shouldn't be there,
and got rid of warning message about decoding '' record.
* Fixed interp option so it now works in dask clusters.
* Better error message when an empty list of files is passed in.

* Added vertical bounds information to --bounds option.
* Removed "coordinates" from internal encoding of xarray DataArray
objects. This makes it easier for the user to remove it from the
attributes in xarray to_netcdf routine.
* Added CCC file interface to the project (work in progress).
* Refactored internal _decode routines to be class methods, which
means the full Buffer object no longer needs to be included in
dask graphs (greatly reducing the size of dask graphs).
* Automatically "fuse" records together in xarray interface, which
increases the chunk sizes in the dask graphs (making operations work
more efficiently in dask).

* Fix compatibility issue with older versions of numpy, triggered
when using gridhacks options.

* Fix handling of accented characters from vardicts and metadata
files.
* Removed 'axis' attribute from 2D lat/lon fields, since they are not
truly axes in that case.
* New Python helper function (fstd2nc.extra.get_crs) for getting
projection information from an xarray dataset.
* Turned off annoying librmn messages when using this module from
Python.
* Some support for creating a new Buffer object by instantiating from
an existing Buffer, allowing more specific filtering options.
* Fix filtering for etikets that contain spaces.
* Detect and transform netCDF variable names that contain a '/'.
This can happen, for instance if there are multiple typvars and one
of them contains a '/' character.
* Some support for complex data (datyp=8) from Python interface.
Note: not supported for writing to netCDF, just xarray interface.
* New to_fstd() method to write back to FSTD format instead of netCDF.
* Use only "current" definitions for --opdict and --vardict options,
ignore the obsolete ones.