Implement additional HWP commands

[BrianJKoopman]

We need additional commands implemented in the hwp module. See this issue about the Supervisor Agent, specifically:

Also, do you think we should implement gripper.grip, gripper.ungrip, supervisor.enable_driver_board, and supervisor.disable_driver_board in sorunlib for the same reason?

There was also some discussion about changing hwp.set_freq() to hwp.spin() to make it more clearly an action that will cause the HWP to start up if it is stopped. The hwp module isn't really in use downstream yet, so now is the time to change it.