When preempting the "exe_path" action we would like to come to a graceful stop and then say we succeeded in cancelling.

Current implementation:

sets the cancel_ flag op true before we can handle any actions in our overloaded cancel function.

This results in an immediate zero publish on the cmd_vel topic.

Desired behavior:

Allow for a custom implementation upon receiving a preempt on an "exe_path" action. For example: We want to adhere to deceleration limits and still follow the current active path. So it could also take some time before the cancel is acknowledged.