Signal output on fuel left

[morse]

It would be nice to have an additional signal output for the entities with fuel meters: to see how full is the meter, it could be a percentage or a simple control signal fuel/no fuel.

The usecase is for gleba: since the fuel (nutrients) spoils fast it would be nice to provide it only when the meter is empty. Another usecase is an alarm for important buildings (heaters, reactors, etc).