continue modularization

[elear]

We are coming to the point where we should do registrations and callouts, like the following:

if WANT_FDO:
   create['fdo'] = fdo_create_device
   delete['fdo'] = fdo_delete_device
...

and then in scim.py:

for a in create:
   create[a](...)

For models.py, we probably want to move things like fdo back to their own scim_fdo files, etc.