Add multifit_solver binding

[guillaumegomez]

Add multifit_solver binding

[SuperFluffy]

With gsl 2.2.1, the follwing change was introduced:

** completely rewritten nonlinear least squares module,
including support for large problems; the user may
now control the linear solver used, the trust region
updating strategy, and the scaling method. In addition,
support has been added for the geodesic acceleration
step (Transtrum 2011) which can speed up convergence
on a wide class of problems.

I figure this is what gsl_multifit_nlinear.h is for. At the same time, they have removed any mention of gsl_multifit_nlin.h from the docs. Here is a mailing list question about the same issue that went unanswered: https://lists.gnu.org/archive/html/help-gsl/2017-06/msg00052.html