Implement TB6612 driver for Raspberry Pi

[climategadgets]

Adafruit DC & Stepper Motor HAT for Raspberry Pi - Mini Kit provides a very reproducible and standard stepper controller solution for Raspberry Pi, it would be a shame not to support it.

This enhancement is much more complicated than #6, though. Steppers, unlike servos, do not have inherent limits, and if a stepper is used as a servo, there will have to be solutions put in place to allow limit detection (limit switches and torque sensors, to name a couple). In addition, stepper positioning model (discrete steps) is different from servo positioning model (floating point 0 to 1 with adjustable ranges and limits), so some extra work will need to be done.