Newton has too many dependencies
The Newton solver has quite some requirements on the Assembler and LinearSolver types which makes it harder (though not impossible) to use in a generic setting. These dependencies are documented in test/nonlinear/newton/test_newton.cc where the minimal current interface requirements are implemented in Mock classes and a scalar nonlinear equation is solved.
This issue is here to remind us that these dependencies should be reduced to a minimum to decrease coupling in the code.