[SIGMON-629] QHDA improvements

Context

• task: https://its.cern.ch/jira/browse/SIGMON-629

What was done

• timestamp_ref added to the _events dataframe and to the Result
• timestamp_ref printed for the main circuits and ipq, ipds and its
• label for the overall result removed
• ipqs, ipds and its: consider the last point with the lower sampling as the beginning of the discharge. (To mitigate the 'lost point' issue)

TODO

• ?

QA

• test are passing

Review objectives

• ? assessment of code functionality
• ? assessment of code quality
• ? identifying potential refactorings / logic common to other projects
• ? knowledge sharing
• ? getting help to understand a piece of code

Closes SIGMON-629