Make Init, Enable, Disable and setPWM of drivers virtual

[Moddingear]

This add the Virtual keyword to functions defined in BLDC6PWM, BLDC3PWM, Stepper2PWM, and Stepper4PWM, so that they can be overloaded further down the line with a call back to the original functionnality.

Use case for this is in the instance of the STSPIN32G4: Bootstrap capacitors need to be charged before any high level output can be requested to avoid triggering Vds protection by high side UVLO.

This is done by enabling the low side mosfets for a time (as described in the datasheet). Overloading the Enable functions allows for that to be done when needed, instead of all the time, as enabling the low side mosfets effectively brakes the motor.

The base class, FOCDriver, already has them virtual, so this adds no extra cost/vtable complexity.