Immediate/override hysteresis working correctly

On user input to the device via the temporary temperature override or
immediate (aka hold) functionality, the hysteresis function should not
prevent these inputs from taking effect. The hysteresis function
prevents unwanted automated heater on/off commands from taking place too
rapidly.

It is not designed to prevent user input from cycling the heater on or
off whenever the user requests it. The code now reflects this design
requirement.