Assess impact of single vs. double precision on performance

In an effort to get across-the-board speedups, we may want to assess the impact of using single vs. double precision. If there is a big performance impact, we may want to consider using single precision at least in certain performance hot-spot areas of the code, if the impact on model results is minimal.

Mariana suggested this November, 2015. From doing a bit of reading, my sense is that this is very processor-dependent (which makes sense).

One area where we would likely see an across-the-board gain due to a switch to single-precision is in memory use: 1/2 the memory use (in areas of the code like the history file) means greater cache efficiency... and it feels to me like a lot of our troubles in CLM are due to cache inefficiencies.