Maxcube: Support On/Off commands to 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
Merge branch 'new-maxcube-onoffsupport' of https://github.com/cyclingengineer/openhab into cyclingengineer-new-maxcube-onoffsupport
Merged PR #897 from branch 'cyclingengineer-new-maxcube-onoffsupport'
introduced constants for DEFAULT_OFF (4.5) and DEFAULT_ON (30.5) temperature