Fstd2nc

* Fixed regression where some data records were being identified as
(unused) coordinate records, causing them to be omitted from the
netCDF file.
* Metadata can now be applied based on forced-renamed variable names.
E.g., can have sections [SSH_vgrid0] and [SSH_vgrid3] in the metadata
file to control the attributes separately for each vertical
coordinate type of an SSH variable.
* Fixed the handling of strings in --filter option. It should now work
with normal (not byte) strings without any padding.
* Also added 'np' shortcut to filters to allow numpy functions to be
used in the filter criteria (such as np.isin).

* New 'fstdump' command for displaying the structure of an FSTD file
in the same layout as would go into netCDF.
* Some support for 'M' and '' grids.
* Improved startup time for the converter by avoiding imports of large
packages like pandas, xarray and dask unless actually needed.
* Removed pydap interface (no known use case, extra work to maintain).
* Much better threading performance for xarray/dask interface.
* Fixed 'interp' argument when used from Python interface.
* Internally, changed Buffer._headers to be a dictionary instead of a
structured array. No impacted expected for user-facing functions.
* Fixed a crash in vcoord mixin when encountering a degenerate '0 hy'
level.
* Also, removed the above-mentioned '0 hy' coordinate from the output
file (treat as 2D field).
* Improved the speed of querying multiple FSTD files by using multi-
processing.
* Added new --serial option to disable the aforementioned multi-
processing.
* Added proper depth coordinate for WSOL and ISOL surface fields, with
new option --soil-depths to override the default values.
* Added support for missing values encoded using the datyp+64 flag.

* Changed the earth_radius grid attribute from 6371229 to 6370997, to
be consistent with librmn.
* Modified the pressure parameterization to be defined in units of hPa,
for consistency with the surface pressure (P0) field.
* Added 'formula' attribute to all vertical coordinates. Note that
this attribute is not part of the CF standard.
* Added support for more vertical coordinates (1002, 5100, 5999, 21001,
21002).
* Support for SLEVE coordinates (and corresponding C coefficients).
* New command-line options --momentum-levels (--mlev) and
--vertical-velocity-levels (--wlev) to subset the data on those
levels.
* In metadata files, allow an attribute to be explicitly *removed*
from a variable by omitting a value after the '=' delimiter.
Useful for turning off attributes that were automagically added
somewhere else in the code.
* Use 32-bit precision for integer values from the metadata files.
* Fix bug in from_fstpy, which was causing a crash if the resulting
dask arrays were sliced.
* Also updated from_fstpy to handle fstpy dask arrays constructed from
compound operations.
* Fix bug in to_fstpy - deleted records were being included in the
final table by mistake. This would cause problems if those records
were ever accessed.
* Allow variable names that already start with an underscore. Do not
prepend a second underscore to them.
* Bug fix in detecting direct grids with 1D lat/lon coordinates.
* Buf fix in extracting lat/lon fields for some trajectory data.

* Handle case where series of points have the degenerate (length 1)
dimension of ni,nj transposed between the data and lat/lon fields.
* Fix crash when decoding a polar stereographic projection when the
pole is outside the domain.
* Fix decoding of INFO record for MLDPn (should be interpreted as text).

* Various bug fixes for from_fstpy and to_fstpy methods.
* Fix duplicate grid_mappings when converting data from GEM LAM grids.
* Check for vertical velocity diagnostic level in addition to usual
momentum / thermodynamic diagnostic levels.

* The --exclude option now works on attribute names.
* Add axis="X" and axis="Y" attributes for all projection types.
* More compatibility updates for Python 3.
* Add "grid_mapping" metadata to degenerate file (when no data records).
* Axis and coordinate attributes can now be controlled with the
metadata-file structure.
* New functions .to_fstpy() and .from_fstpy() for interoperability with
fstpy package.
* Fix warning message with 'O' grids.
* More user-friendly suffixes when splitting variables across different
coordinate types (e.g. model_levels/diag_level and
depth_levels/bottom_level).
* Better error handling for malformed / unsupported vertical !! records.