Exploit einops for docs / tests / advertising

[mcabbott]

When last I checked this package did everything they do. Would be nice to

• See if they have weird edge cases which make good tests
• Translate some of their tutorials, either just as a source of structure / pretty pictures, or as an explicit guide to learning X if you know Y.

Xref #46 (comment)

Their docs: https://einops.rocks/#tutorials

Their paper: https://openreview.net/pdf?id=oapKSVM2bcj

Old notebook here: https://github.com/mcabbott/TensorCast.jl/blob/master/docs/einops.ipynb