Make linked_fields a dictionnary of additional fields.
Old behavior (reusing a form field) can be specified by setting the field to None.
Fix miscellaneous stuff:
- rename CompoundModelFormSet to CompoundInlineFormSet to match Django terminology.
- make the grouping field setting an OrderedDict of fields to add.
- add factory for formsets.
- replace assertions by a custom exception so it can be handled.
Implement proxy formset tests.
Remove redundant is_valid and has_changed methods.
Bypass subform initial value with common initial value form linked fields.