Calibration on non-linear totals

[haroine]

Following the linearization formulas of Dell et. al: http://www.crest.fr/ckfinder/userfiles/files/Pageperso/xdhault/doc_JMS.pdf, Icarus could support calibration on non-linear totals such as quantiles, Gini indices, etc.

Due to the approximations, we would have to come up with an iterative procedure that would check that the final estimates correspond to the true value.

Some functions of the gustave package https://github.com/martinchevalier/gustave could be reused