Refactor Phase/WalkCycle/Trajectory

[dellaert]

Currently a Phase/WalkCycle/Trajectory are specifications of intervals. I propose to re-factor in the following way:

• A trajectory is a collection of phases (applies to both walking, jumping, rocketry)
• A phase is an interval with a ConstraintSpec (e.g., feet on ground, but could be other things)
• a new class, FootContactState, is a ConstraintSpec specific to walking
• A walk cycle is a vector of FootContactStates, that can spawn phases. Repeatedly spawning phases is a way to create a trajectory.