ChargeBattery task with finish_time earlier than start_time

[Yadunund]

The task planner uses A* search to find a set of assignments for all robots such that an overall cost is minimized. The cost here is the sum of the differences of the estimated finish_time and given earliest_start_time for each task. Every time a new node is evaluated, ie, an unassigned task is assigned to an agent, we check if a ChargeBattery task should be added before/after. It is expected that anytime a task is added, the total cost of the node will increase as the finish_time for any task will always be greater or equal to the earliest_start_time.
However, this is violated in an edge cases due to a bug in the implementation of ChargeBattery::estimate_finish(). Here, when determining the time to recharge the battery, we first compute the battery_soc after the robot has reached its charger. Then the required increase in soc from the current value to the agent's recharge_soc is determined. However, if the user set the recharge_soc to a small value such that it is below the soc of the agent at the charger, a negative duration for recharging gets computed. There is an assert in place but it doesn't have any effect in release builds.

As a result the finish_time could end up earlier than the earliest_start_time. When this happens, adding a ChargeBattery task to the node actually lowers the cost. Hence this assignment will remain in place while other requests get allocated accordingly.