Gaz pressure evolution in the tank atmosphere

The Tank module today only allows for fixed pressure atmosphere. It would be useful (for exemple for the mini-simulators project) to model the pressure like in a gas volume.

Some hints:

• The gas volume could be included in the Tank, potentially as a replaceable component (default=constant, replaceable with a volume?).
• The gas volume must evolve following the level in the tank.

Extra: potentially, variable sections at different heights could be also implemented (but maybe this is a little too specific).

Potentially this development will be somehow obsolete with multi-fluid support... but maybe it will still be a low complex approach for some "simple" models (not needing the burden of multi-fluids).