Resolve "[frictionlaws] Roughnessheight calculation in friction laws"

Make adding artificial water depth in friction laws optional:

TODO:

• adapt limitRoughH in frictionlaw.hh for zero roughnessHeight
• make roughnessHeight optional parameter (default = 0.0) for Manning
• make roughnessHeight optional parameter (default = 0.0) for Nikuradse
• Improve documentation in friction laws

Add some tests

• Add test Nikuradse

Notes for the reviewer

I added a new rough channel example with limited Nikuradse friction law to ensure that the changes will work. All previous tests and examples produce the same results since the water depth was always large enough or no friction law was applied.

Should these changes are mentioned somewhere?

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Closes #1216 (closed)