Alter models + form/json deserialization so that the type enum is
a parameter field. This means it is visible in the default JSON
serialization.
Add more meta models
Mid refactoring, basic machinery in place for exposing a 'client'
format Json for meta-models in a generic manner. Lots of work still
to do in making it neater and more comprehensible.
Tweak error handling
Add abstract to IsadG fields
Show abstract in favour of scope-content where it exists
Merge branch 'master' into nested_meta_model for
added 'abstract' field.

Conflicts:
	app/models/DocumentaryUnitDescription.scala
JSON tweaks
Do meta model client formatting manually, because that allows us to
put the model fields at the top level, which is clearer. It's not too
much extra typing either, since the meta models have few fields.
Fix address deserialization
Started elaborating the structure of the metamodel with base traits