EV3-G API: motors (#360)

* EV3-G API: motors

* EV3-G API: motors - fix things from code review

* Make MoveSteering a child class of MoveTank

* rename power to speed, is a % of max_speed, must be between -100 and 100

* Remove _set_position_rotations call in _set_position_degrees

* rename max_speed_pct to speed_pct

* Comment out logging in core.py

Author: dwalton76