refactored the MapStopDecision function in StGraphMapper

distance

Thanks to @haoyang

and decisions

main decision can be abstracted into a stop obstacle
Mission Complete should be processed in high level code, such as frame
or planner

This reverts commit c0eb3348.

* it is not an Impl of a pure interface. Impl is misused here
* The class is more like a collection of functions, which can be
improved.