Clean `apollon.tools.standardize`

[Teagum]

Thetools.standardize function computes the z-transform, or z-score of the input array. The name standardize is, hence, misleading. I should rather call it 'z_score', or 'standard_score'.

Todo

• rename the function
• Improve the doc string the give more information on what is a z score.
• find all calls to it and update them
• Write unit tests