Decouple CompoundGenerator behaviours

CompoundGenerator performs 2 functions: nesting generators and creating all the points for a scan.

With the addition of Zip and Concat generators, the nesting behaviour should be removed from the the creating points generators, as these generators must work on Generators (which a CompoundGenerator is not). These models also hint that there could be further ways of combining two generators (although I'm struggling to think of geometries other than a inside b, a alongside b, a after b)

This would allow CompoundGenerator to become another normal Generator, which can be wrapped by a Zip/Concat, and could allow a model to be populated with points without a wrapping generator, which would make the matching of Java and Python concepts of generators in GDA a little simpler.

Cons: pretty large rework
Pros: separation of concerns, allows nesting Compounded models within other 'Multimodels'