Improvement DCCompensation Pulse

Because of reflection in types.py, we cannot use the original idea of extending SquarePulse (class DCCompensationPulse(SquarePulse)), because when added to the scheduler it will try to parse the arguments given to the init function of DCCompensationPulse rather than SquarePulse.

To fix the above, I moved the init to a static method: create_compensation_pulse, that returns a SquarePulse.

I have also fixed some bugs and added documentation regarding sampling_rate.