Make SimpleMotorFeedforward only support discrete feedforward

[narmstro2020]

This PR sets period as a constructor argument along with adding another constructor overload to accomodate.
The constructor now creates the feedforward, plant, r and nextR variables used by the calculate(current, next, dtSeconds) overload.

The calculate(current, next, dtSeconds) overload is now calculateDiscrete(current, next).
The calculate(velocity, acceleration) overload is not calculateContinuous(velocity, acceleration).
The calculate(velocity) overload is unchanged. I'm not married to these name changes.

I will eventually move on to Elevator and Arm and C++ for all 3 based on the feedback I receive.