Integrate LinearSegmentedColormap in ColorManager

Description

As @tgaskin brought up, it would be nice to be able to define a custom colormap by specifying discrete colors between which the colors are interpolated. The LinearSegmentedColormap allows for that. Currently the ColorManager only supports the ListedColormap for truly discrete colormaps.

Proposal

Allow to define any matplotlib.colors.Colormap via the ColorManager.

How to test the implementation?

Extend existing tests.