Improving ModelArrayRef

[timspainNERSC]

Currently ModelArrayRef has two outstanding problems:

1. The set of possible referents is limited to whatever is listed in the ProtectedArray and SharedArray enums.
2. There is an additional dereference to get the position of the referenced array in the big array of pointers.

The main issue with problem 1 is that it limits the ability of future researchers to add new fields that can be shared by the ModelArrayRef mechanism. If a future researcher wishes to add young ice parameters, then they would have to add HICE_YOUNG, CICE_YOUNG &c. to the enum in order to use the ModelArrayRef class to share the data throughout the model.

If the key used to access the referenced data was a string, then the value could be much more arbitrary, albeit at the risk of name clashes. If the key is being changed to a string, then since the underlying store is to be changed, the additional dereference could be eliminated.

[timspainNERSC]

The code to fix this is present in the branches containing the substring mar3 and their descendants.

[timspainNERSC]

Closed by merging PR #250