Migrated .plot() from using xarray

[jakubchlapek]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Summary

Changed Timeseries.plot() implementation to no longer rely on xarray under the hood while keeping same functionality.

[dennisbader]

Beautiful, thanks a lot @jakubchlapek 😍 🚀

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.45%. Comparing base (9b61374) to head (4e98c5f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2932      +/-   ##
==========================================
+ Coverage   95.40%   95.45%   +0.05%     
==========================================
  Files         146      146              
  Lines       15701    15703       +2     
==========================================
+ Hits        14979    14989      +10     
+ Misses        722      714       -8     

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[dennisbader]

I added some unit tests for TimeSeries.plot(), will merge after all tests have passed.