Feat/create package

See merge request !4

Feat/symmetrization

See merge request !3

- Make r13 as default to have be†ter readability
- Needed a change in lienar solver
  - Because indefinite system yields different solution with mumps, now use umfpack
- In general, for non-inflow conditions, we have non unique solution because of pressure
  - Solvers can therefore differ in results (is bad)

- Not used fo now
- Maybe in future because error "peaks" when not antisym

- To allow for better view of symmetry

- See documentation for explaination
- Test cases had to be adapted for promille changes

- Derive the used identities

- This was anyways only working for heat
- Makes no sense to keep this option anymore