Need to import a reference to fstd_core for the refactored fstd.save() to work.
Fix other parts of code which were looking for 'deet' in the NPAS axis, or putting it there.
When recreating an NPAS axis, check if there is no forecast (single value of 0).

For this case, use an npas axis of zero (don't need to know about deet).

This resolves a divide-by-zero issue for some analysis fields, which have
deet=0.
Only warn about encoding a generic axis if the original axis is some unknown subclass of ZAxis.

Don't warn if it really is a ZAxis, since it's already generalized before we
process it.
Fix a bug when encoding lat/lon axes back to fstd.

The lambda expression for data_func didn't bind the xcoord/ycoord, so all
lats/lons ended up using the same data_funcs as the last coords.
Fix fstd_core.encode_loghybrid_table - was encoding the wrong 'skip' value.

The skip value is an offset along the nj dimension, not including the ni(=3)
dimension.
Allow the same datyp / nbits to be used when writing back to an fstd file.