Towards a more consistent DUMUX course for version 3.1

All issues that came up during the dumux course in Tunis or which I noticed:

• exercises-properties: A "TODO" component might be added in neumann() to be consistent with other exercises

• exercises-runtimeparams: update Readme.md. The default input file is now params.input, not "executablename.input", or more correctly, both are working by default.

• exercise-coupling-ff-pm general: Many of the files do not contain "TODO" comments where changes need to be made in the fashion of the other exercises.

• exercise-coupling-ff-pm (a: interface and b,c): warning about neumannFlux being deprecated, probably should be updated.

• exercise-coupling-ff-pm interface: I guess that the tasks in the Readme.md should include removing the initial velocity field in initialAtPos().

• exercise-coupling-ff-pm (b: models): the 2p2c model's indices.hh is referenced in the Readme.md. However, this does not seem to exist.

• exercise-biomineralization: The line numbers in Readme.md do not match the current code. eps_ is added both for the upper and lower injection area boundary. A comment in Readme.md should be added that NH4 is ignored in the simplified biomineralization model and thus also the chemistry.

• general comment: there is always the "property macro deprecation warning" when compiling any of the course exercises. Are the exercises free of the property macros?

• general comment: The names of the exercises' executables could be more consistent, e.g.: ex_coupling_* vs exercisebiomin

• general comment: deprecation warnings for gravity

Resolves #21 (closed)