Added capability to process the add_offset attribute in input NetCDF files

[arango]

Added code in the NetCDF input routines to process the add_offset attribute, if present in an input NetCDF variable. This implies that users need to be careful when including such an attribute in a NetCDF variable.

If the add_offset attribute is present for a variable, its numeric value is added to the data after is read by ROMS I/O interface. In some applications, the add_offset is used to provide simple data compression. Also, it can be used to change units. For example, we can convert temperature from Kelvin to Celsius:

        float Tair(time, eta_rho, xi_rho) ;
                Tair:long_name = "surface air temperature" ;
                Tair:units = "Celsius" ;
                Tair:add_offset = -273.16;
                Tair:time = "time" ;

Here the original data is actually in Kelvin, so a value of -273.16 is added to the data to convert it to Celsius.

Many thanks to Kate Hedstrom for suggesting this capability.