Add altitude and relief colormaps

[juseg]

Add five colormaps from cartowik, document them, and enable them in plot methods.

• [x] Add altitude and relief shading colormaps.
• [x] Enable new colormaps in plot methods as imshow.
• ~Add color table example as in matplotlib.~
• [x] Add colormap reference as in matplotlib.
• [x] Document new colormaps on plotting page.