Configure task planner to always append a desired task at the end of the plan

[Yadunund]

It is desirable to have the task planner automatically append a desired task at the end of its plan (while ensuring there is sufficient battery to perform it). This is useful in scenarios where we may want an agent to always go back to its charger after completing all its tasks. Or go to a holding point. Or do something else.

Architecture wise, we can modify the plan() method to optionally accept a factory that will "make" a Request. Internally, we can call this make with the appropriate start_time after the planner finishes generating a plan for the original set of requests. Then we can then rerun the planner for the newly generated request while using the final state of the agent from the previous plan. The results can be appended to the original and returned.