Add Tests

Add different (unit/system) tests and test cases to verify the implementation.

Ideas:

• test that parameters are set and initialized correctly
• test subsolvers:
  • Richards: steady state, no river height changes -> solution should not change
  • Richards: Newton and L-scheme should give the same result
  • Richards: example with varying soil types? (#7 (closed))
  • SWE: steady state case?
  • SWE: benchmark?
  • SWE: switch time stepper?
• test coupling:
  • generate precice-run.xml from params
  • explicit coupling should run without errors
  • implicit coupling should run without errors
• numerical tests?
  • verify first order convergence of full scheme
  • benchmark example?