Beam and frame FEA models

[yijiangh]

Adding beam and frame models to the TrussTopOptProblems (we should rename it by then), so we have a "full model" for linear elements in 2D/3D that includes not only axial forces but also bending and torsion. The efforts will be in:

1. Extending dofs to include angles.
2. Extending our element stiffness matrix to translate my python code here. We can debate to directly hardcode it or try to write it in a Ferrite-friendly way (like what we've done for truss elements).

And that's pretty much it!

It would be interesting to try reproducing results from this paper: https://www.sciencedirect.com/science/article/pii/S0045794917300639