Model objects should be in CamelCase.
Add a Model type hint.
Remove is_object() calls when the test subject is already type hinted as a Model.
Update DboSource::group():

* Update documentation.
* Add Model type hint.
* Bail early when fields are empty.
Add more Model type hints, and ease the model testing replacing is_object() for  '!== null'.
Merge pull request #2458 from bar/2.5-model-typehints

Add Model type hints, and ease testing for Model.