Implement getMaximumExtent

[jeking04]

Implement the getMaximumExtent function on aikido::ompl::GeometricStateSpace. This is used by OMPL planners to compute collision checking resolution. We are avoiding this by defining a custom MotionValidator. But there may be other uses of the function that could cause planning errors.

[jslee02]

There is a WIP branch for this: feature/ompl_max_extent. It would be nice if someone takes over it.