Quantitative trends: add feature switch all/complete cases

[TammoReinders]

For the qualitative trends it is especially of interest to gain the option to have a look at all collected data. This would require the following sub-tasks:

• Add a switch, that controls if only the values of those participants are shown, that have no missing values for the selected visits. This is referred to as complete case analysis. This should be the default. Upon switch, every available value for all selected visits should be shown, regardless of the missing values in the other visits. This is referred to as all cases analysis
• The test options to test for differences between visits are paired tests. For the all cases analysis, non-paired test alternatives are required.