unify color styles across plots

We could revise the color schemes used in various plots and unify them a bit. Maybe have an "AIXD" palette with main colors to use.

main TODOs:

• create aixd cmaps (divergent, monochrome, categorical)
• tone-down plot backgrounds (now: bluegray --> white or grey)
• move color settings from mlmodel to visualization module

smaller TODOS:

• in Plotter.kde_distribution_attributes - make vertical lines (mu, sigma) the same color as the attribute - cannot distinguish if multiple overlapping plots
• Plotter.distrib_attributes - 10/90 marker lines not red
• apply to sensitivity analysis plots