Don't duplicate entity type dictionary in MooseModel cache

[NicolasAnquetil]

MooseModel has a storage containing a dictionary byType

And it also manages a dictionary of entities by their type to answer messages like allClasses, allMethods, ...
through the call to allUsing:ofGroupClass:

This is a duplication of logic which is harmfull.

We need to remove the dictionary in cache and allXYZ should use the storage (MooseGroupStorage) instead of the cache