[Feature Request]: Add higher order fit for RDTs

[Mael-Le-Garrec]

Feature Description

RDTs are calculated via a fit of a line amplitude relative to their action part, that varies with kick amplitude.
Currently, this fit is done for each RDT separately, meaning we can't differentiate the contribution of RDTs on the same line.

Possible Implementation

A higher order fit with all the RDTs contributing to the same line in rdt.py L227
And calculating the linearized problem for all those RDTs at once, in rdt.py L233