Control option to adjust setpoint relative to default

[mnblonsky]

Right now it's easy to control HVAC and WH setpoints to an absolute value, e.g.:

wh.update({"Setpoint": 50})

It's a lot harder to control to a relative value (relative to the default). I think this is the best way to do it now (this varies slightly if the setpoint is defaulted or if it's specified in the schedule):

t = wh.current_time
setpoint = wh.schedule[t, "Water Heating Setpoint (C)"]
wh.update({"Setpoint": setpoint - 5})

Maybe it's worth adding a new control option so that we can do this in 1 line? Maybe "Setpoint Change" or something like that?