lfi2cdf: is now able to create new variables from read variables.

         Example: lfi2cdf -v CLD=RCM+RIM+RSM,WT ...
                  Will write in the netCDF file 2 variables: CLD which is computed
                  as the sum of the RCM, RIM and RSM variables (read from the LFI file)
                  and the WT variable