ITC503: investigate logic

The gas/heater mode on an ITC503 is a combined entity, so to set a new gas mode you need to know the current heater mode. For the unknown quantity the logic will use the :SP variable unless it is in alarm (undefined) then it uses the readback. I am not sure of the history if this, normally we would use the device readback as otherwise e.g. if you changed the heater value on the device front panel it would get overwritten by the last gui heater:SP when you changed gas flow from the GUI

There was an issue on WISH where the gas flow could not be changed, this may have been due to an issue in this logic as it will not set the combined gas flow/heater values if it thinks it does not know one of them.

## acceptance criteria

* confirm logic and change as necessary