Allow user to set image colormap

[kadrlica]

@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.

[kadrlica]

Also pinging @jdswinbank since this is an example for a community PR.

[kadrlica]

Usage example:

# Use lsst.afw.display with the matplotlib backend
import lsst.afw.display      as afwDisplay
afwDisplay.setDefaultBackend('matplotlib')

plt.figure(figsize=(10,10))
afw_display = afwDisplay.Display(1)
afw_display.setImageColormap('gray_r')
afw_display.scale('asinh', 'zscale')
afw_display.mtv(calexp.maskedImage)
plt.gca().axis('off')

which produces

[jdswinbank]

Thank you for this! I've moved it onto a DM branch in #8, and have asked @RobertLuptonTheGood to review it there.

Since this is a small patch, I don't think it's enough to give you a copyright claim on the code. However, if you disagree, let me know and provide an appropriate copyright statement (see https://developer.lsst.io/legal/copyright-overview.html#copyright-holders for more).