read data source from global attribute for IC and LBC

[Jili-Dong]

If users encounter the following errors during FV3 initialization:

fv_regional_bc::get_data_source - input source not found in file gfs_data.nc.
or
fv_regional_bc::get_lbc_source - input source not found in file gfs_bndy.tile7.000.nc.

that means the global attribute "source" is missing in IC (gfs_data.nc) or LBC (gfs_bndy.tile7.000.nc). The accepted FV3 source is one the following:

"FV3GFS GAUSSIAN NEMSIO FILE"
"FV3GFS GAUSSIAN NETCDF FILE"
"FV3GFS GRIB2 FILE"

Users can add the attribute to gfs_data.nc or gfs_bndy.tile7.000.nc using the NCO command:
ncatted -O -a source,global,c,c,'FV3GFS GAUSSIAN NETCDF FILE' gfs_data.nc
or
ncatted -O -a source,global,c,c,'FV3GFS GRIB2 FILE' gfs_bndy.tile7.000.nc

or consider to modify the IC/LBC generation code to write out the global attribute "source" as a long term solution.