1) the name of the meshmask file to be reduced. In that case,
there is only one meshmask file
OR
2) the xxx part in the names: xxx.mesh_hgr.nc xxx.mesh_zgr.nc
xxx.mask.nc. In that case, the meshmask is split into 3 files.
ncfileout
inoptional
the name of the reduced meshmask file. default
definition is smallmeshmask.nc