Missing solids4foam

[MakisH]

We need this for our new perpendicular-flap/solids4foam tutorial case.

[MakisH]

This actually needs OpenFOAM v2012 (older) at the moment, so I think it is fine to not ship it this time and wait for solids4foam to support the latest OpenFOAM. I can add a note in the release notes. We also only support an unreleased version at the moment.

See the installation instructions: https://solids4foam.github.io/installation/installFromSource.html