Add PP load + save support for potential-temperature vertical levels.
Review changes.
Merge pull request #886 from pp-mo/tpot_levels

Add PP load + save support for potential-temperature vertical levels.
Add PartialDateTime
Merge pull request #885 from rhattersley/partial

Add PartialDateTime
Added options for fileformats.netcdf.save

Adds new options for saving netCDF files to allow for
custom unlimited dimensions and saving netCDF4 files with
compression.

See discussion in #860.
Changed unlimited dimensions to use coordinate names

Sorry about that!
Fixed failing unit tests