Rework the Op contract documentation and add a bit about __props__.
Fix imports for the documentation.
Fix documentation warnings in sparse/basic.py
Remove section titles in the docstring (they don't show up and cause warnings).
Fix most of the remaining warning for the doc.
Add mentions of __props__
Reflow paragraphs
Remove flops() since it's not really important.
Remove warnings about nonlocal images.
Changes following review.
Add mention of __str__ to the __props__ blurb in the tutorial.
Merge pull request #2069 from abergeron/doc

Doc