Clean-up system classes and evolution storage types

[pietercollins]

The system classes and the related set types should be cleaned-up. Issues are:

• Make sure all systems, including Map and VectorField have named variables, state and auxiliary variables.

• Make HybridSpace an abstract class allowing for Monolithic and Composite spaces.

• Clean-up HybridGridTreePaving to be a map from the location to a labelled paving.

• Consider changing HybridGrid to be a fixed set of locations, or make HybridGridTreePaving use HybridScalings (and maybe HybridSpace) directly.

• Amend Enclosure and Storage classes to have named variables.

• Adapt Enclosure and Storage classes to conform to a common interface.

• Change Enclosure and Storage classes to use more general Set or Paving objects.

• Change Enclosure and Storage classes to require the auxiliary mapping and state/auxiliary variables in construction.

[lgeretti]

I believe this could be better served by a milestone to which you associate all the relevant single issues (and we already made issues for some of these). Unless you believe you can address all these in one run.