Removed duplicate call to setPlanningScene(), added various comments

There's just two lines in this PR of functional change - I found that the setPlanningScene() was being called twice and, looking into the functionality, it is unnecessary. I moved the ``context->setMotionPlanRequest(req);`` call down with it for better grouping.

There will probably be some more similar pull requests soon, but I'm going to try to keep them small for ease of merging.