Allow Results objects to work with Estimators' fit() method

[tsalo]

Closes #52.

Changes proposed:

• Overwrite Estimators' .dataset_ attribute with None when calling fit(). fit_dataset() will then replace that with the actual Dataset if it is called. Otherwise, .dataset_ will remain None.
  • This allows us to initialize Results objects, which assign an attribute (.dataset) from Estimator.dataset_.
• Check if Result.dataset is None before using it. When it is None, raise a ValueError.
• Add warnings to Result method docstring about whether fit is okay or if you need fit_dataset.
  • This way, it is clear when fit-generated Results' methods can be used and when you need a fit_dataset-generated Result.
• Test the new behavior.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.60%. Comparing base (541894e) to head (afb605d).
Report is 27 commits behind head on master.

Files	Patch %	Lines
pymare/estimators/estimators.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   87.44%   87.60%   +0.16%     
==========================================
  Files          13       13              
  Lines         844      863      +19     
==========================================
+ Hits          738      756      +18     
- Misses        106      107       +1     

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[JulioAPeraza]

I left one small comment. Everything else looks good to me!