Add sparse matrix support for some matrix plotting functions

[bdpedigo]

• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• Have you built the documentation (reference and/or tutorial) and verified the generated documentation is appropriate?

Reference Issues/PRs

None

What does this implement/fix? Briefly explain your changes.

• Allows the user to pass sparse matrices to adjplot and matrixplot when using the scattermap option

Any other comments?

• Could absolutely have a conversation about the best API for these functions. I'm not currently that happy with it, but don't want to let that hold up this PR.

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[bdpedigo]

TODO:

• Update documentation
• Add a test

[bdpedigo]

@nyecarr any chance you'd be willing to have a look at this since we were just talking about it?

[bdpedigo]

@asaadeldin11 any chance you could take a quick look at this? No worries if not just lmk

[asaadeldin11]

Looks good to me @bdpedigo .

[bdpedigo]

thanks @asaadeldin11 !