Coerce tau2 to an array in permutation_test

[tsalo]

Closes None, but should fix a bug when running permutation_test on WeightedLeastSquares results, since WLS has a single tau2 value across all datasets.

Changes proposed:

• Ensure that tau2 is an array within permutation_test, so that it doesn't fail on 1D Datasets.
• Use MetaRegressionResults.permutation_test() in an example.

[codecov-commenter]

Codecov Report

Merging #94 (aeeffe7) into master (cda03cc) will decrease coverage by 0.07%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   87.51%   87.44%   -0.08%     
==========================================
  Files          13       13              
  Lines         841      844       +3     
==========================================
+ Hits          736      738       +2     
- Misses        105      106       +1     

Impacted Files	Coverage Δ
pymare/results.py	81.59% <85.71%> (-0.23%)	:arrow_down:

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