rocking curves: add an option to select one of the fitting method

Today only "trf" fit is used for fitting rocking curves.

Looks like other method could be faster and work in several use cases.

The goal of this PR is to allow users define this method and investigate if this is relevant or not.

This is related to #122 but depending on outcome maybe we will need to find other ways. To be discussed with @maciej.jankowski