Support for goal TSRs

This implements goalset TSRs. Trajectory wide constraints and start sets not yet supported.
Explicitly storing active indices in `_indices` makes `RobotSize` double the size of an equivalent state in a `RealVectorStateSpace`. Assuming that the parent `RobotStateSpace` outlives all of its `RobotStates`, we could pass the indices by reference or pointer. Does OMPL guarantee that a state space outlives its children?