ufl description of trusses in 3D

[srosenbu]

Currently trusses are hardcoded and the stiffness is added to the stiffness matrix directly. It would be nice to have FEniCSX do this work.
FEniCSX currently does not support mixed meshes and therefore, one of the difficulties will be to define the problem on two meshes but still have the dofs in the same Linear system of equations.

[srosenbu]

Here is an example where 1d-elements are included in a 3d space in legacy FEniCS
https://comet-fenics.readthedocs.io/en/latest/demo/beams_3D/beams_3D.html