Add a function for merging observed flows from a same site

When running get_hubeau_flows using a the code of a site, one could retrieve several columns corresponding to several stations existing on this site.

It would be interesting to have a clean function that merge these time series using the algorithm used in Nunez Torres (2021).

In case of full periods with duplicated stations, it would be interesting to take into account the "qualification" indicator https://id.eaufrance.fr/nsa/515 to weight each station depending on the data confidence.

@guillaume.thirel, have you ever done this kind of task before? Do you already have some smart algorithm about this?

Reference

Nunez Torres, Laura. 2021. « Simulation d’un bassin versant anthropisé à l’aide d’un modèle hydrologique semi-distribué : Le bassin de la Seine et ses réservoirs Rapport de stage ST5 -Polytech Sorbonne -1er septembre 2021 ». Other, PolyTech Sorbonne. https://hal.inrae.fr/hal-03359617.