Fix bug in eta A values for Hybrid levels.

pref is in units of hPa, so need to convert to Pa for the formula.
Release 0.9.1.
When encoding back to FSTD, put the 'deet' value in the field atts, not the NPASAxis atts.
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.
Release 0.9.2.
Merge branch 'release-0.9'