Add shaded relief plot methods

[juseg]

Add shaded relief functions from cartowik, and enable hillshades in either existing, or new, accessor plot methods. Try to keep reasonably low numbers of arguments in plot method.

• [x] Add hillshade functions from cartowik and docstrings.
• ~Rework *_altitude plot methods to include hillshade, or~
• [x] Add {bedrock,surface}_hillshade plot methods.
• [x] Add mat and shiny shaded relief examples.
• [x] Include a shaded relief on plotting page.