Find plateau

[Didou09]

Motivations:

• Users handling multiple time traces may want to identify plateau phases in them.
• This feature would be particularly interesting after using uniformize() to find all plateau phases on a common time basis, then using plot_BvsA_as_distribution() for statistical studies, or exporting to pandas DataFrame

Suggestions:

• Contact Jorge MORALES (IRFM) to get the details on how his linbrary works
• Create a new dedicated branch from devel, call it Issue47_Plateau, work only on that branch
• only work in the dedicated module datastock/_find_plateau.py
• Use this issue ticket to exchange on how to do the implementation