Use matrepr for rich __str__ and _repr_html_

[alugowski]

If available, use the matrepr package to render array values in string and HTML output.

Reverts to current behavior if matrepr is not installed.

See this Jupyter notebook for a demo of what 1D, 2D, and 3D sparse arrays look like when rendered by matrepr.

Closes #604

[hameerabbasi]

Fairly minimal and self-contained. Thanks!

[hameerabbasi]

I'll take a look at the CI later today.

[alugowski]

I'll take a look at the CI later today.

I see a few other recent PRs with the same fail.

The message is right, .pre-commit-config.yaml does not appear in the repo, though I see some PRs don't run that check at all.

[codecov]

Codecov Report

Merging #605 (e5b06b6) into master (096f9b0) will decrease coverage by 0.21%.
The diff coverage is 59.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
- Coverage   92.15%   91.95%   -0.21%     
==========================================
  Files          20       20              
  Lines        3313     3330      +17     
==========================================
+ Hits         3053     3062       +9     
- Misses        260      268       +8     

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[alugowski]

The test fails were from the linter, I implemented its suggestions.

[hameerabbasi]

LGTM, thanks!