message: changed from argmin_derive to argmin_codegen
diff: #[macro_use]
  extern crate argmin;
  #[macro_use]
  -extern crate argmin_derive;
  +extern crate argmin_codegen;
  use argmin::prelude::*;
  use argmin::solver::gradientdescent::*;
  // use argmin::solver::linesearch::BacktrackingLineSearch;
  