pre-process incoming CaptureBase to CaptureMotion

Add ProcessorMotion::processIncomingCapture function.
Called at the very beginning of the process scheme, it converts the input CaptureBasePtr (_incoming_ptr) to a CaptureMotionPtr. The default implementation remains exactly the same while allowing derived classes to overload the newly added processIncomingCapture function.

Use-case :
A Scan-matching-based ProcessorMotion receives a LaserCapture. It thus has to compute the actual scan-matching in order to estimate a MotionCapture. It therefor takes place in processIncomingCapture.
While this may not be ideal, it is the simplest/easiest hack around this problem.

Note, I thought I had added this a while ago, maybe not.