Also some of the smaller suggestions of the review process are
integrated.

- use std::array instead of std::vector.
- In general manningN is a spatial parameter, so put it there, even
  if it is a constant in the rough channel test.
- Rename the water depth from h to waterDepth.
- Fix the dambreak test, which was incidentially broken by the work on thei
  rouch channel test.

The test provides a simple channel flow based on the friction law after
Manning. Normal flow is assumed, therefore the formular after Gaukler,
Manning and Strickler can be used to get an analytic solution.