ovito, imageio, Pillow as optional dependencies

[jonathanfischer97]

Optional dependencies

• I was working on this before pulling your latest changes @sikaoguo22 beating me to the punch by simply just removing imageio and ovito as dependencies, but in my implementation here I tried to make them optional.
• So if a user has them installed in their environment they can still use the visualization methods that import those packages.

pytest target

• Added tests as the target for pytest in project.toml, which seems like an improvement because without a target, pytest seemingly tries to run all scripts that have *test* in their name, at least for me.

Remove or make optional? I'll leave it up to you @sikaoguo22 or @yingyue2030699

• Not sure whether it's best to simply removing the deps like in the latest HEAD, or allowing their option like in my PR here.
• The optionality certain adds some complication, but I included a test script that ran successfully and at least tested that the package can function without ovito, imageio, or Pillow installed.
• The entire package ran successfully with pytest before I committed these changes.

[codecov]

Codecov Report

Attention: Patch coverage is 8.82353% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ionerdss/nerdss_analysis/analysis.py	8.82%	31 Missing ⚠️

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