Add warning when OMP_NUM_THREADS > 1 or is unset

[arbennett]

@jhamman recently noticed that he was oversaturating Cheyenne when running with -n 36. It was determined that OpenMP was at least part of the issue. It appears that something in MetSim uses it under the hood.

It seems that if OpenMP is available on the system MetSim will attempt to make use of it. We should do a quick check in the driver if the environment variable OMP_NUM_THREADS is unset or if it is greater than 1. If the variable is unset, the library defaults to use OMP_NUM_THREADS = #processors.