Try to improve performance of routines in subgridAveMod

The routines in subgridAveMod get called a lot. Improving their performance by even a small amount could significantly improve performance of the whole model.

Before spending much time on this, we should put timers in these routines to time the total amount of time spent in them, to see if this is indeed a worthwhile endeavor. (However, we may need to be careful about this, because I've experienced that putting timers around such small blocks of code can lead to significant time being spent in the timing routines themselves.)