DOF index vs. joint index mismatch

The current implementation around `or_ompl::GetContinuousJoints` [here](https://github.com/personalrobotics/or_ompl/blob/c156a88386e98baace2f0010aaaf1b7deac0eefb/src/OMPLConversions.cpp#L90) conflates DOF indices with joint indices, and will therefore fail on robot with multi-DOF joints.
