Hi,

I just added a new nonlinear solver for the steady command: it is a
trust-region solver, available under solve_algo=4 (it replaces the old
value of that option, which was a Newton without the correction for
singular Jacobian; I had added that variant a long time ago, and it was
neither really useful nor theoretically grounded).

The implementation of the solver is essentially the fsolve.m from
Octave, that I have adapted for MATLAB compatibility and that I have
also slightly simplified.

I am wondering whether we may want to use this new solver as the
default, since trust-region algorithms are nowadays considered to be the
state of the art for solving systems of nonlinear equations. Said
otherwise, should we modify the default to be solve_algo=4 instead of
solve_algo=2 ?

Since I have not thoroughly tested the new solver, a first step would
probably to test it on various relevant examples.

Best,