Maxcube: Support On/Off commands

 Support on off commands for radiator thermostats and correctly support 0.5 deg increments.

ON command will set radiator thermostat to full 'On' (i.e. 30.5 deg). 'On' is displayed on unit
OFF command will set radiator thermostat to full 'Off' (i.e. 4.5 deg). 'Off' is displayed on unit

Also fixes issue where we were casting double to int then multiplying by 2, resulting in losing the .5 values. This was stopping On/Off setting as they are 30.5 and 4.5 respectively