Custom colormaps

[anderskaestner]

Is your feature request related to a problem? Please describe.
I'd like to load custom colormaps in TomViz.

Describe the solution you would like
Please add a "Load colormap" button in the colormap selection dialog and provide documentation on the file format for such colormaps.

Describe alternatives you have considered
Add some documentation on how a new colormap can be added to the Select colormap dialog

Additional context
This feature request is a workaround to allow visualization of combined bivariate data.