Add "compare" step

[annennenne]

This is a planned extension of dataMaid. Feel free to add suggestions or comments.

Together with the three existing dataMaid data screening steps (summarize, visualize and check), we will add a new step: compare. The current data screening steps only look at variables one by one and therefore, it is not possible to accommodate row-wise comparisons/checks. This will be possible to do in the "compare step". Here, the user can specify comparisons, e.g. linear restraints (variableA > variableB).

We are considering the following:

• Support specifying comparisons with rules from the validate package.
• Allow users to specify comparisons class-wise. E.g. for all factor variables, check that they are not identical to variable a.
• Allow users to specify comparisons for specific variables only. This would be very flexible, but less straight forward to make user-friendly.
• No default comparisons should be included, but a few different comparison functions should be implemented and readily available.

Issues that are related to this extension and will be solvable afters its implementation:

• Online tool feature wish: check for unique identifiers [Online tool feature wish: check for unique identifiers #41]
• Connection from the identified outliers to the row or unique identifier [Connection from the identified outliers to the row or unique identifier #15]