Added ability to return used planning context in planning pipeline

This adds a new feature to the planning request adapter chains that tracks the used planning context throughout as the various adapters call each other in succession. It allows one to pass in a context pointer and when the chain returns it keeps the planning context in existence for further usage and analysis. I'm using this to allow me to access the OMPL SimpleSetup class from within the planning pipeline, so that I can call some custom functions on it that I need.

This change also requires the planning adapters to be slightly modified to accept the context in their function headers.

Finally, I renamed two functions to improve clarity and added some inline documentation.
Good question. This commit is over 3 months old, so I do not recall why I have that there or if it should be there. I've gone ahead and fixed it because I think you are right. Side rant: we really need to be more responsive in merging stuff if we want contributors to MoveIt! (and also thanks for reviewing this now, better than never)