Add isValidSetpoint()

[kcooney]

This is a proposed extension point for the problem that motivated the changes in #93.

• Define a documented extension point (isValidateSetpoint()) that allows subclasses
  to determine that a provided setpoint is invalid
• Update setSetpoint() to document that it will do nothing if isValidateSetpoint()
  returns false

Note: I went with this change, instead of making setSetpoint() non-final, because
if subclasses could override setSetpoint() they could do almost anything (including
passing a different setpoint to super). These changes provide an API to allow the
actual need (discarding values that are determined at runtime to be invalid).

[spderman3333]

I'm not sure if this is really necessary, as couldn't this be implemented in the subclass.

[kcooney]

I'm not sure if this is really necessary, as couldn't this be implemented in the subclass.

@spderman3333 we reverted #93 since Le Prawn wasn't extending MotorSubsystem anymore.