Add non-linearity correction

At PDR we were asked to not only flag but also correct non-linearity.

Trello board issues copied into comments below.

Mailed Bogna Kubik bkubik@ipnl.in2p3.fr for info about Euclid tests for non-liearity and persistence on 2018/12/03: https://mail.google.com/mail/u/0/#search/to%3Abkubik%40ipnl.in2p3.fr/QgrcJHrjCsmZcrCWrjHjFGcVBhvQcblMPRb

Made living document: https://docs.google.com/document/d/1LFQRE2LcuhbPfuaG00Oa_wjN7kwIjvzIqUQpd4rCBd4/edit#heading=h.a5p75sav8an5