Test setting absolute pressure to 0 in FlowSystem

[mwetter]

The challenge problems in Fluid.Examples.FlowSystem have small pressure drop differences but large absolute pressures.

This issue is to test the effect of setting the absolute pressure to (near) zero.

[PeterMeisrimelModelon]

Unless I misunderstood something, this would allow negative pressures, since the pressure variables now represent the deviation from p_default = 300000 ?

I ran into a lot of bounds issues when trying to simulate, but simulation works just fine when ignoring them.

Would one need to adjust the minimum of the pressure variables as well?

[mwetter]

I don't recall running into issues due to negative pressures on this development branch. But if some are triggered then yes, the min attribute of the pressure can be removed as this branch gets negative absolute pressures. (The media still works fine in this non-physical region.)