use udunits to check units

[burlen]

in a number of algorithms the units of scalar fields or coordinates are crucial. the netcdf files typically provide units for all data. we've been checking this manually using string comparison, and we don't handle many valid unit formats. udunits can decode these and has comparison methods. some places where we make such checks:

1. integrated vapor transport
2. integrated water vapor
3. TC MPI
4. normalize coordinates

this is likely an incomplete list, and other algorithms should probably have checks. grep for '.get("units"', '[ *"units" *]', etc