#12 scaffold tree view: make gradient range of property mapping configurable

[Till Schäfer]

Wish:
The gradient range of the property mapping for the scaffold tree view should be configurable.

Current behavior:
The two gradient colors selectable are chosen for the maximum and minimum (accumulated) value of the unfiltered dataset.

Problems:
- If there is an outlier, the practically used range of colors can be very small. Therefore the visually differences are two small which is leading to meaningless mappings.
- The same problem occurs if the current visible subset has very little variation in contrast to the whole dataset. However sometimes it is needed to have the ranges to be the same over different views with different subsets, because it makes two different views comparable. Therefore choosing min and max values for the subset only as the range is not a solution to the problem.
- Sometimes only a particular range of values is interesting and should be mapped to the colors

Proposed solution(s):
1. Make the range for gradient mapping configurable. All colors above the maximum (minimum) value are either mapped to the maximum (minimum) color or to a different (third) color.

2. Merge the interval and gradient mappings in the following manner.
-> Always use intervals. Delete gradient mapping.
-> Modify the interval mapping to allow a start and end color for each single interval. When choosing different colors make a gradient between this tho colors for the range of the interval only. If both colors are the same, the interval settings are equivalent to the current interval mappings. This solution is much more flexible than the previous solution (1). It allows multiple gradients with hard cuts between interval borders for a single property. It is also possible to combine gradients with "traditional" interval mappings.

Additional:
For both solutions it would be nice to have some quick-settings which adjusts the current mappings to the min/max values of the current dataset OR subset.

[Lina Humbeck]

I like the solution 2. Two things I miss in the current gradient settings are:
configuration of the colors of the gradient
setting a min/max value (additional use case see below)
A min/max value would also be useful for the heatmap as there each property has its own range but for comparison a common range would be desirable.
Both points should be fixed with the suggested solution 2.