Variable data redesign

[timspainNERSC]

It is increasingly clear that there should be only one copy of variables in nextSIM-DG, and that should be the copy with the full set of DG components. This will require redesigning how data is held in the model and how it is accessed by the thermodynamics and dynamics components.

The design should met the following requirements:

• The use of DGVector in the dynamics is paramount. The speed of the dynamics core is crucial to the execution speed of nextSIM-DG.
• There should be minimal changes to the API of ModelArray.