[Fluid] Pressure conditions

[rubenzorrilla]

This adds the possibility to set custom pressure conditions similarly to the custom velocity constraints. So far I only made it available for points in 2D and points and lines in 3D.

@jginternational we need to confirm that conditions are never written for this. In other words, we only need to write nodes in the submodelparts to which we assign this condition.

[rubenzorrilla]

It is currently set as: 2D -> Point & Lines -> Nodes 3D -> Point, lines & surfaces -> Nodes

If it's fine for you, you can merge

Provided that only nodes are written in the corresponding submodelparts it should be fine.