- Renamed register_diag_field() to register_diag_field_low().
- Added wrapper register_diag_field() to call register_diag_field_low().

TODO:
[ ] Add optional arguments to provide CMOR names.
[ ] Add multiple invocations of register_diag_field_low() with the
    various different names that might appear in a diag_table.
[ ] Do same for static fields.

- Added wrapper to register_diag_field() to store second handle
  for CMOR registration.
- Added wrapper to post_data_*() to call send_data for CMOR handles.
- Alternative handle is stored in diag control structure.

… not present

- Attribute fall-back logic for CMOR scalar fields
- Wrapped register_Z_tracer, added standard_name attribute
- Added CMOR names for the following diagnostics
   o  areacello (static)
   o  so (3D)
   o  so_z (3D-remapped to Z)
   o  tosga (scalar - in the style of a CMOR scalar diagnostic)
- Added entries for cmor fields to the diag table (global ALE Z example)