Forward model Geopotentials

[Huw Lewis]

GRAS SAF helpdesk enquiry from Axel:

When running the ropp_fm_bg2ro_1d with ECMWF hybrid levels I noticed
 that geopotentials are required as input. These geopotentials are
 actually calculated by the forward model from the model levels and 
are correctly written out into the output file (independent of the 
actual input geopotentials). So it actually makes more sense to 
remove them from the input requirements and only write them out.

The availability of valid 'level' data (impact, geop_refrac, geop) are used in ropp_io_rangecheck to test whether Level1b, 2a or 2b data exist in a file. Even if no rangechecking is applied, the number of geop data points in a file (valid or otherwise) defines the size of the Level2b structure on reading a ROPP input file.

For FM applications, it does make sense to have geop as only an output variable (since it is not actually required as an input to FM calculations). More generically, defining each data structure based on the available height parameters (and time for Level1a) is the most generic method.

Ticket created to review this as part of ROPP-4 development.