WIP: change preprocess_orca, so that it behaves as intended in the example

The example states:

'''
This is meant to be used like:
ds = xr.open_mfdataset(
     data_files,
     preprocess=(lambda ds:
                 preprocess_orca(mesh_mask_files, ds)))
'''

If one wants to pass data_files only to xr.open_mfdataset() and mesh_mask_files only to preprocess_orca(), the resulting ds does not contain mesh- and mask-variables. If these should be contained in ds, 41cfa4a4 is needed.