Change default colormap for the outputs

[leofang]

From @hanfeiyan:

Also, I don’t like the default colormap for the display (Xiaojing agrees). Is it possible to make it selectable (just like DPC)? If that will take too much work then I’d prefer either bone or gray as the default color.

This will be a two-step change:

1. Simply change default colormap
2. Make the colormap selectable

The second step needs to touch the GUI....