[swe][frictionlaws] Enable test for Nikuradse

One can run the test now also with Nikuradse. But be aware that the
friction value in the input file is a Manning values.
Addintional changes
- The names of the variables used for the calculation of the
  boundaryfluxes are changed, because they were not really clear. Also the
  documentation of their file is improved.
- Some smaller modification were also applyied, which were demanded
  by the discussion about the merge request.