Fstd2nc

* Fix bug when reading multiple files with masked records, when some
records are repeated across files.

* Fix bug when there's a mixture of 0mb and other pressure levels.
This was triggering some weird behaviour in the 'accum' mixin,
even if the fields weren't accumulated quantities.

* Fix decoding of extended dates, which were off by 1 day.
* Fix for auto-detecting accumulation periods when there are some
missing records.

* Fix missing grid_mapping information for 'A' and 'B' grids.

* Further fixes for some 'Y' grid files, which were being
misidentified as 2D fields after the updates in the previous version.
* Fixed generate case where there is a single lat/lon point in the
file.
* Added documentation for string arguments to --filter option.

* Added quiet=True option for Buffer interface, similar to --quiet
option from command-line.
* Added some 'yin' and 'yang' projection metadata to files on a
yin-yang grid.
* Fixed data type for north_pole_grid_longitude (double instead of
int64).
* Added --decoder option for choosing method of reading the FST files.
(experimental).
* Persist last file opened for I/O, which may speed up accessing
multiple records from the same file.
* Allow fstdump to take multiple input file arguments. Previously
this could only be hacked in with globbing patterns surrounded in
quotes.
* Fix missing lat/lon coordinates for some 'Y' grid files.