Make colormaps configurable for each variable

[sahahner]

Is your feature request related to a problem? Please describe.

For some variables, there are specific colormaps (precipitation, sea ice concentration) that we should also use in the validation pipeline to allow quicker and more intuitive analysis of the results.

Describe the solution you'd like

We should allow the config files to specify colormaps for some variables.

Describe alternatives you've considered

Right now some colormaps are hard coded using if-statements at several places in the code.

Additional context

No response

Organisation

ECMWF