Use absolute colour scale

[peterdesmet]

The vp-color-scale is currently relative, i.e. the maximum colour is assigned to the maximum number in the current data range. This makes it hard to interpret the data:

• How is bioRad doing this? @adokter
• Is it technically possible to implement a maximum in CROW? @niconoe @stijnvanhoey
• Is it possible to define an absolute maximum across all radars? @adokter @CeciliaNilsson709
• Or should we assign a maximum per radar (e.g. in the settings), which might make it harder again to compare radars?

[niconoe]

I don't think there is any technical issue in making the max of the scale fixed. My reasoning at the time was probably that it was hard to decided what the max should be (wide variation across days and radars), so making it dynamic avoided the issue. As you mention, it has a cost: the comparisons are difficult.

In practice, making it fixed will probably allow to simplify the code a bit. We'll have to make sure the D3 scales have the "clamping" feature on, so the values above the max are clamped to it (rather than triggering an error).