Misc API fixes

[rabstejnek]

Some 500 errors were reported during our API scan as follows:

• TypeError at /epi/api/group/
  • Request Method: POST
  • Exception Type: TypeError
  • Exception Value: 'bool' object is not iterable
  • Exception Location: /usr/local/lib/python3.11/site-packages/hawc/apps/common/serializers.py, line 281, in to_internal_value
• TypeError at /epi/api/group-result/
  • Request Method: POST
  • Exception Type: TypeError
  • Exception Value: 'bool' object is not iterable
  • Exception Location: /usr/local/lib/python3.11/site-packages/hawc/apps/common/serializers.py, line 281, in to_internal_value
• KeyError at /ani/api/animal-group/ 'dose_units_id'
  • Request Method: POST
  • Exception Type: KeyError
  • Exception Value: 'dose_units_id'
  • Exception Location: /usr/local/lib/python3.11/site-packages/hawc/apps/animal/serializers.py, line 116, in
  • Raised during: hawc.apps.animal.api.AnimalGroup
    • /usr/local/lib/python3.11/site-packages/hawc/apps/animal/serializers.py, line 116, in
    • units = set([d["dose_units_id"] for d in doses])

The first two errors stemmed from the same bug; we were iterating over data before determining if it was iterable.
The third error occurred when a dose_unit_id was not given for a dose; this was fixed by making dose_unit_id required when deserializing dose data.

[shapiromatron]

LGTM - since you converted the custom validation code to a serializer.Field, I took advantage of that downstream when validating the Dose Regime and removed a little more custom code.