Rework ModelArrayRef

[timspainNERSC]

Given the changes likely required by #154, ModelArrayRef should not be dependent on ModelComponent.

This can be achieved by removing direct references to the ModelComponent arrays of ModelArrays and replacing them with second template argument pointing to a backing store. This will make the variable declaration a little less clean:

ModelArrayRef<ProtectedArray::H_ICE> hice;

will become something like

ModelArrayRef<ModelComponent::H_ICE, ModelComponent::ProtectedArrays> hice;

instead.

Also, the enumeration will likely have to be a trad enumeration instead of a C++ enum class, unless there is something clever to be done with class inheritance there. Sadly.

[timspainNERSC]

Merged