Feature/battery planning

[Yadunund]

This PR introduces a new package, rmf_battery to model the battery drain for mobile robot participants in RMF. In particular it introduces:

• rmf_battery::agv::BatterySystem class which is a container for battery properties of the vehicle.
• rmf_battery::agv::MechanicalSystem class which is a container for mechanical properties of the vehicle.
• rmf_battery::agv::PowerSystem class which is a container for power system/device properties of the vehicle.
• rmf_battery::MotionPowerSink pure abstract class which contains compute_change_in_charge(rmf_traffic::Trajectory trajectory). A sample implementation is provided by rmf_battery::agv::SimpleMotionPowerSink
• rmf_battery::DevicePowerSink pure abstract class which contains compute_change_in_charge(double run_time). A sample implementation is provided by rmf_battery::agv::SimpleDevicePowerSink

The expectation is for the user to use a combination of MotionPowerSink and DevicePowerSink(s) to compute the expected battery drain for various applications

[Yadunund]

Closing in favor of https://github.com/osrf/rmf_core/pull/217