Feature/virtual fields & sourcefieldswitcher

[jarekwg]

1. Introduces SourceFieldSwitcher, a pseudo-field for allowing a variety of different columns to map to the same destination.
   This is super useful for FKs, which you might want to look up by, say, name or ref.

2. Tidies up a lot of the form class creation code into a separate formclassbuilder file.

3. Drops the need for defining "virtual_fields". This was a lazy way of telling DMI that a field was to be present on the form despite not coming explicitly from the import file. FlatRelatedField was already being handled automatically, however JSONField and anything using CompositeLookupWidgets wasn't. Moving forward, all these fields will just work properly without needing to define stuff in "virtual_fields".