OpenMP Speedups

This pull request adds OpenMP multithreading for clustering, building the Chow Liu tree, and FabMap place comparisons.

* It is designed to return identical results to a single-thread (sacrificing some speed in a couple places).
* Enabled automatically in CMakeLists if package OpenMP is found.
* Runs up to 8x faster on quad-core with hyperthreading.
* Added status messages to bowmsctrainer, since this can take a long time.
 * Uses cout right now, maybe a better logging method can be suggested.