Remove alpha parameter of Magnitude-Shape plot and outlier detector.

[vnmabus]

• This alpha parameter was only used in the asymptotic case, when the number of samples was too high.
• Add instead a cutoff_factor parameter that multiplies the cutoff and can be used for an arbitrary number of samples, to consider more or less functions as outliers.

[codecov[bot]]

Codecov Report

Merging #424 (c626fce) into develop (85046fc) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #424   +/-   ##
========================================
  Coverage    80.24%   80.24%           
========================================
  Files           92       92           
  Lines         7229     7229           
========================================
  Hits          5801     5801           
  Misses        1428     1428           

Impacted Files	Coverage Δ
.../exploratory/outliers/_directional_outlyingness.py	91.53% <100.00%> (ø)
...exploratory/visualization/_magnitude_shape_plot.py	68.31% <100.00%> (ø)

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