OpenMP Speedups

[kmactavish]

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.