Added DistanceFieldManager for CHOMP

I added a DistanceFieldManager class to CHOMP that automatically handles syncing distance fields between the environment and CHOMP's internal representation. This is necessary to call CHOMP multilpe times in the same environment. It also completely replaces the old `computedistancefield` function.

This requires the latest version of CHOMP from Github:

https://github.com/personalrobotics/or_cdchomp

Thanks @cdellin for making the necessary fixes on the C++ side!