Add GenericNonlinearSolver interface

[Prof Garth Wells]

As part of the linear algebra clean-up, we should add a very simple GenericNonlinearSolver class that has the absolute minimal interface to solving a nonlinear problem.

A user can pass a concrete implementation of a GenericNonlinearSolver to a solver to gain fine-grained control over the solver.

See https://www.mail-archive.com/[e-mail]/msg02749.html.

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)

[Jan Blechta]

See also #714.

[Jan Blechta]

Consider also #814 when already doing interface changes.