Add "summary" method to Results classes

[tsalo]

References #5, but doesn't close it because I don't know what to do for plotting.

Changes proposed:

• Create MetaRegressionResults.summary(), modeled after metafor's rma results __repr__ (or whatever it's called in R).

To do:

• Review outputs and check their correctness
• PermutationTestResults
• CombinationTestResults

[codecov-commenter]

Codecov Report

Merging #105 (f82482a) into master (f425ff9) will decrease coverage by 0.07%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   87.60%   87.52%   -0.08%     
==========================================
  Files          13       13              
  Lines         863      890      +27     
==========================================
+ Hits          756      779      +23     
- Misses        107      111       +4     

Impacted Files	Coverage Δ
pymare/utils.py	75.00% <60.00%> (-25.00%)	⬇️
pymare/results.py	83.91% <100.00%> (+1.28%)	⬆️

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[tsalo]

I'm not sure what the CombinationTestResults summary should look like given the limited results, and PermutationTestResults's summary would depend on the parent Results type.

I almost think that we should remove PermutationTestResults completely, and just stick with the other two objects.