Simplified logic in cylinder management and gas change events

[Jan Mulder]

Correct the bug that a manually added cylinder get "used" right away, 
and removal becomes impossible. From now on, a cylinder is only 
considered used when there is a gas change event for it. This is 
implemented by removing the "used" flag, and replacing that 
functionality using a function that simply decides whether a cylinder is 
used by inspecting the events.

best,

--jan
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