Metrics clean

New merge request for the metrics.
The code is almost the same as in metrics branch (more PEP8/pylint corrections, more comments).
Functionaly, the main difference is that the API metrics returns a 404 error if the user has no access to the webapp. Previously, it was a 403 error. The need to managed the different cases was not obvious. I choose to simplify.

The commits have been redone, one per new feature. API metrics, tests, doc, metrics html.