Need to document and comment SIMA grid/map routines inherited from CAM.

### Issue Type

Documentation Update

### Issue Description

CAM-SIMA inherited a large number of Fortran modules, DDTs, and subroutines designed to handle various model grids, and their mappings to various PIO decompositions. While they seem to generally work fine, the original developers who created them have left, and recent experience has found that it is somewhat difficult to understand what everything does, and what the correct course of action is when something new needs to be added.

Thus it would be good at some point for an engineer to sit down and closely examine what all this code is doing, and to add comments and documentation so that any future work will be easier to implement, especially for new developers who are generally unfamiliar with this part of the model.

### Will this change answers?

No

### Will you be implementing this yourself?

Any Software Engineer can do this