DM-15182: Add facility to change matplotlib colormap

[jdswinbank]

No description provided.

[jdswinbank]

See also commentary on #5.

[kadrlica]

Moving some comments/questions from #5...

@rhl this is my first attempt at implementing a facility to allow the user to set the image colormap (#1 and DM-15182. Some notes:

• I've implemented both public (setImageColormap) and protected (_setImageColormap) methods with the idea that the public method would be eventually moved upstream to afwDisplay. (I'm not sure about the naming convention because the preceding function, show_colorbar, does not seem to conform with other methods in Display.)
• I've inserted a stand-in for testing this facility in test_display_matplotlib.py. The test is currently commented out because I'm not sure whether we want to handle the creation of the display in setUp.

[jdswinbank]

Review comment from @RobertLuptonTheGood:

I don't think I want to pass a pyplot.cm.XXX object to setImageColormap; so display.setImageColormap("gray"). If you agree (or can't convince me to the contrary) I'll make that change. It would be nice to come up with a subset of commonly supported names; at the very least "gray"

(Also some follow up Slack discussion, which I'm not recording here.)

@RobertLuptonTheGood — I think it'd be helpful if you can make further comments here, rather than Jira, just to save me from copying & pasting. Does that work?