missing option to disable auto-masking in readers

Since netcdf4 v1.4.1 all output from a netcdf file will be a masked numpy array also if there is no missing data.

This breaks workflows at the moment since some regridder are not compatible with unmasked data.

See here: http://unidata.github.io/netcdf4-python/#netCDF4.Variable.set_always_mask