add relative humidity to specific humidity calculation for GFSv15/16 grib2

[JiliDong-NOAA]

The current relative humidity to specific humidity conversion in chgres_cube (rh2spfh) is not consistent with GFSv15/16 grib2 files (when spfh is converted to rh by UPP in operations). A more consistent rh to spfh calculation could help to keep consistency when processing GFSv15/16 grib2 files (thanks for the discussions with Guoqing Ge of GSL, Fanglin Yang and Wen Meng of EMC).

Furthermore, when there are missing levels of spfh (e.g. GFSV16 combined grib2), using rh with full levels could be a better option over using spfh with interpolation to missing levels.

A PR will be submitted shortly to add two namelist entries:

1. a parameter to choose if using rh or spfh in grib2 files for humidity ("use_rh" = false as default).
2. a parameter to choose which rh to spfh calculation will be used. ("calrh" = 0 as default to use the current calculation, =1 to use calculation consistent with GFSv15/16).