* Split low and high level controller methods

The Controller class is growing to be pretty bulky, and its methods
can generally be split into two categories...

- Low level methods that simply mirror what the control-spec provides.
- Higher level methods (like the get_relay() and get_all_relays()
above) which are more user friendly and build on what the control spec
provides.

Ravi: Do you have any thoughts on this? Do you think this would be a
good way to break up the controller, or would this just make it more
confusing? Alternatively we could wait until we try using stem in arm
to see if this makes sense or not...