provide simple beam simulation for unit testing

[joergbrech]

Proposal: In the sense of TDD:

• Create dev branch @joergbrech
• Add a simple FEM beam simulation, e.g. using fenics @joergbrech
• formulate a simple optimization problem that has an analytic solution @joergbrech
• write a unit test that compares results to analytic solution @joergbrech

Then, in a new issue,

• @alxbusch implements everything in dev (or a branch created off of dev) until unit test succeeds.