Femwell

- Add function to calculate intensities
- Add function to plot intensities

- Fix meshing if a LineString is cut by the geometry

- Update minimum version of scikit-fem

- Fix n_eff guess to make scipy solver work

- Add mesh refinement
- Add scipy benchmarks
- Fix n_eff-guess for compute_modes (->seems to fix mode calculations with scipy)

- Rewrite maxwell waveguide mode solver - now completely object oriented!
- Start implementing schrödinger's equation
- Start docs for schrödinger's equation
- Add potential well