Add optional display of derivative curves in RelPerm plot

[magnesj]

Add derivatives as curve selection option. Consider adding a special axis for derivative curves, as the range will be independent to the values of the original curve.

Investigate how the the user interface can be designed to make it easy and intuitive to use.

Use the tool from #11837 to compute the values for the derivative curves

Also consider display of scaled /unscaled in the same plot #11684