A python interface to UNIDATAs Udunits-2 library with CF extensions. Store, combine and compare physical units and convert numeric values to different units. Units are as defined in UNIDATAs Udunits-2 library http://www.unidata.ucar.edu/software/udunits/, except for reference time units (such as 'days since 2000-12-1' in the 'proleptic_gregorian' calendar), which are handled by the netCDF4 python package. In addition, some units are either new to, modified from, or removed from the standard Udunits-2 database in order to be more consistent with the CF conventions http://cfconventions.org/.