Add ReadablePIDOutput to the project.

This class implements a PIDOutput that can be read by an outside user.
Add methods to AnalogAbsoluteEncoder and its subclasses.

The new methods allow AnalogAbsoluteEncoder to be easily zeroed and
fix rollover detection in the encoder.
Delete out-of-date swerve code.

The swerve code is still in development is in a different branch.