Separate DiscreteSINDy as a new class

[yb6599]

This PR separates DiscreteSINDy as a new class that derives from _BaseSINDy following the discussion in #351.
Also fixes #650.

[yb6599]

The scikit-time tests fail as the scikit-time API has remained unchanged. How should I proceed with this? @Jacob-Stevens-Haas

[Jacob-Stevens-Haas]

Part of the problem with the face-to-face conversations is that I can't remember them.

Refresh my memory. What was the issue removing the parameter discrete_time from the scikit-time classes?

[yb6599]

Removing discrete_time should not be a problem here. However, we had not decided how we would deal with the discrete time implementations of the scikit-time API.

[codecov]

Codecov Report

❌ Patch coverage is 96.10390% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.74%. Comparing base (472e5a2) to head (a8eeb56).

Files with missing lines	Patch %	Lines
pysindy/_core.py	95.97%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #654      +/-   ##
==========================================
- Coverage   95.03%   94.74%   -0.29%     
==========================================
  Files          38       38              
  Lines        4106     4148      +42     
==========================================
+ Hits         3902     3930      +28     
- Misses        204      218      +14     

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[yb6599]

Do we need _BaseSINDy.print() if we have SINDy.print() and DiscreteSINDy.print()?

[Jacob-Stevens-Haas]

90% of the way there, thanks for this long work!

Few changes, few nits, few documentation/test things

[yb6599]

Ready for final review and merge.