PR Summary

register_cmap() could be used either with a Colormap, or by passing data to implicitly create a colormap from.

This PR deprecates the second way in favor of explicitly creating colormaps viaregister_cmap(cmap=LinearSementedColormap(name, data lut)).

Having both ways is redundant, makes the function more complicated, and unnecessarily pulls colormap implementation details (like thelut) into the API ofregister_cmap(). Given that colormap registration is only done very rarely, the benefit of a sligtly shorter callregister_cmap(name, data=data, lut=lut) is negligable. It's even error prone becausedata has to be passed as kwarg since the second arg is the for this case unusedcmap parameter.

PR Checklist

• Code isFlake 8 compliant
• Documentation is sphinx and numpydoc compliant
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way