Units module to work for the case of 'tos' in degC

Hi @vagudets

This merge request is to solve this error:

Error in units_transform(data[x], orig_units = orig_units[[x]], user_units = user_units,  : 
  Transformation unknown from degc to c

that happened when reading 'tos'. I was trying to use the SEAS5.1 in gpfs.

I have included a check for units "degc" to be considered as "c".

Cheers,

Núria