add isheating as propery of water heater

[qunh]

add isheating as propery of water heater by utilise the function of isHeating () in the instance
plus requirement file update

[ross-w]

Thanks again for your efforts on this. Have you tried this one in a development environment? I'm not sure just adding it as a property would expose anything to HASS (as isHeating doesn't appear to be part of WaterHeaterEntity.)

My understanding is that it either needs to be a new sensor, or in this case, it is probably more relevant for it to be added to extra_state_attributes. That said, I haven't been following the new features in HASS too closely, so maybe I missed some magic auto enrolment thing.

Also, I'm pretty sure (at the very least) it will need to be added to the update method, otherwise isHeating will only ever be updated when HASS first starts up, and not whenever anything changes