Calibrate minimum speed.

[yohell]

• Goal: determine constant MIN_POWER, being the lowest motor power that will still cause the wheels to move.
• Motivation: Trying to turn or move very slowly does not work, and is an issue when iterating toward a desired direction/position.
• Method:
  ** Drive forward or turn while decreasing power. Observe cutoff where movement stops.
  ** Enter values straight into code.