ependent on
                   the variable being interpolated (u and v need lonc,
                   latc whilst temperature and salinity need lat and lon).
                   - Mobj.siglayz - sigma layer depths for all model
                   nodes.
                   - Mobj.ts_times - Modified Julian Day times for the
                   model run.
   hycom       = Struct output by get_HYCOM_forcing. Must include fields:
                   - hycom.lon, hycom.lat - rectangular arrays.
                   -