Remaining cleanup for the new Rust solver

[stfnp]

Continuation of #276

• Have a look at all the TODO comments in the code, fix important ones and keep rest for later (Prepare a release of the solver on crates.io #278)
• Rename _eval to _sample for all the geometry stuff
• Evaluate the beam's lumped inertia as defined in the theory manual
• Find a clean solution for evaluating layer stresses and strains only over the length of the layer. Same for layer thickness plots.
• Unify the geometric data that is part of the simulation output with the geometric data queried and used by the UI.
• Create an abstraction for static step size control and use in bracing as well as static simulation
• Expose the remaining numerical simulation settings in the bow model (Newton settings, static step size control)
• Implement linear stiffness matrix more generally as a combination of rotation and selection of dofs