Issues with Source.Clearance parameters

[dietmarw]

When looking at Sources.Clearance there are several issues that could be improved:

1. Order: SolutionFlow, Clearance, K and useSolutionFlowInput should be grouped together
2. Dependencies: If K is needed to fulfil Clearance=K*solutionFlow then why can this not be calculated but needs to be provided
3. Units: AmountOfSolutionIn1L really is a very complicated way to define the concentration. Why not simply use concentration and make use of the SIUnits.Conentration type. This is in mol/m3 but can simply be used *1000 in the code. That way the unit reflects what it is and the name is not as unfortunate.