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
More revisions per PR discussion
Merge pull request #863 from shoyer/netcdf-save

Added options for fileformats.netcdf.save including enhanced unlimited_dimensions support.