Make the swing and stance controller interfaces identical

[Nathan Kau]

Put the swing proportion calculation inside the swing controller.

StanfordQuadruped/src/Controller.py

Line 54 in 61144b1

 self.gait_controller.subphase_ticks(state.ticks) / self.config.swing_ticks 

[Steve Macenski]

Or better yet, create a pure virtual controller interface these adhere to.