Add 'add offset' option to getdata

[salwood82]

Some data sets require an offset to be added to certain variables (e.g. SST and salinity in the global HYCOM reanalysis). Currently, this has to be added by the user following download, but could be added as an input parameter in nest_x.nml as with velocity_conversion_factor.

Note that the HYCOM server also states that the data (all variables) require a scaling factor of 0.001. This appears to be automatically applied on download, while the offset value is not. For example, the fill value of -30000 stated on the server becomes -30 . This means that a fill value of -30 and velocity_conversion_factor of 1 must be given in the nest_x.nml file (see example at expt/input_1/nest_1.nml).

HYCOM uses the NetCDF Climate and Forecast (CF) Metadata Conventions detailed here: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.0/build/cf-conventions.html