[ENH] add option to assume constant mask from first time-step or no-mask

NetCDF supports time varying masks for variables and by default the netcdf4 Python package always returns masked arrays.

Since regridder in FINAM require a constant mask, the NetCDF reader should provide an option to assume a constant mask from the first time-slice of a variable.

At the moment we always set Mask.FLEX for the mask attribute of an output.