Improvements for the biomin exercise

• Unit in biominproblem.hh line 201 should be kg/s not m/s. - [ ] Add an overview with the names of the chemical compounds and the corresponding molecular formula in the description.
• Inconsistent use of rurea / r_urea in simplebiominreactions.hh, lines 102 and 103.
• Source term for H2O should be -2*rurea. If this is neglected in the solution, a comment would be helpful. Also comment on ammonium/ammonia which is neglected in this exercise, but present in the reaction equations.
• Task 2: inconsistent use of subscripts between Readme and chemistry file
• Task 2: cleanup the chemistry, e.g. use const Scalar volFracBiofilm = max (0.0, volVars.solidVolumeFraction(BiofilmPhaseIdx))
• Task 3: More info on how to implement stuff in the source() function?
• Task 5: cleanup neumann(), e.g. remove unnecessary "else"? Move pre-prepared "const Scalar gasFlux" to the top of the function below waterFlux and comment on the units? Maybe add some more info for the task (or in the Readme)?
• Task 6: in Readme: remove code on "factor" but add return refPerm * pow((poro)/(refPoro), exponent); to make the "suggested" exercise spatialparams.hh to be identical to the solution.
• Trim down slides to 10 min. Maybe remove some of the "specific implementations"? Or some of the introduction?