Fstd2nc

* Some support for new RSF file container and fst24 data format.
* Fixed bug in fstpy bridge - nk no longer stored in Buffer.
* Fixed check for operational dictionary when opdict=True is used in
Python interface.
* Added _CRS attribute to xarray interface.
* Fixed bug when using dask distributed - the keys were not integers,
but rather numpy integers, which apparently was a problem.
* Some support for mask typvar '!'.
* Fix potential bug if used in future librmn release 20.1 - the
call signature for armn_compress now requires an extra 'swap_stream'
argument to control byte swapping of the decoded values.

* Fixed a bug in open_index() when the index contains pickled data.
* Fix some bugs in vertical bounds (when --bounds is used).
* Added some more fields to the list of accumulated quantities.
* Fix a bug when using .from_fstpy() followed by .to_xarray() on
a single record.

* Fixed bug in make_index / open_index when storing scalar parameters.
* Fixed bug in handling large files with masked fields. The mask
addresses were stored in 32-bits, causing an overflow in the address.

* Fixed preferred chunking when creating xarray objects via
open_dataset().
* Reduced the memory footprint of Buffer objects, and some small
speedups in the header decoding.
* Fixed CCC decoder. It was broken after a recent refactor, and there
was bug where the min value wasn't being added to the decoded
values.
* Added xarray backend for CCC files, so they can be opened with
xr.open_dataset(...,engine='ccc').
* Catch some possible runtime issues when fetching data from xarray
interface, such as if the file was removed from disk. Return
missing values if a problem occurs.
* Improved efficiency of --fstd-compat option (less padding used).
* Fixed a bug when using --bounds on surface data that has a
'sfc_type' dimension.
* Silence some warnings from recent vgrid libraries, when querying for
the existence of certain parameters.
* Added an 'accum' axis for accumulated quantities. Also added an
'--accum-vars' option to explicitly set fields that should be
treated as accumulated.
* Explicitly use a spherical globe in the projections returned by
fstd2nc.extra.get_crs().
* New helper methods for working with ridiculously large datasets
with xarray.
- fstd2nc.Buffer.make_index() pre-scans the input files and
generates an 'index' file on disk which contains all the variable
structures.
- fstd2nc.Buffer.open_index() reads the above 'index' file and
returns an xarray Dataset representing the FSTD data. This step
should be *much* faster since the files were already scanned
in the first step.

* 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.