Allow planning context to be saved in planning pipeline

Modifies the planning pipeline's ``generatePlan()`` function to accept an optional planning context parameter by reference so that the user can get a pointer to the used planning context.

Also modifies all the ``planning_request_adapter_plugins`` to accept the new ``context`` parameter as changed in https://github.com/ros-planning/moveit_core/pull/182/files
Since this is probably an end-user entry point, and we don't know if someone is using the API, and we already have a version of generatePlan with just scene+req+res, it might make more sense for there to be a new generatePlan with context (thus, three generatePlan functions, all eventually calling the new one with all parameters).