Renamed eta to eta-squared in power functions

[raphaelvallat]

Closes #268

[codecov]

Codecov Report

Merging #280 (c1da906) into master (742f930) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #280   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          19       19           
  Lines        3287     3287           
  Branches      523      523           
=======================================
  Hits         3254     3254           
  Misses         17       17           
  Partials       16       16           

Impacted Files	Coverage Δ
pingouin/power.py	100.00% <100.00%> (ø)

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