Python nonlinear solvers interface memory management

[Jan Blechta]

NonlinearProblem and NewtonSolver contain virtual methods that

1. take tensor arguments by reference
2. are supposed to be overloaded by user in Python

SWIG directors create no-deleter shared pointers from references. This is not good.

Possible solutions:

1. Implement a check into SWIG directors checking that user did not store the pointers.

2. Change NonlinearProblem and NewtonSolver C++ interface to take shared pointers instead of references. Note also NewtonSolver::solve(GenericVector& x) would need to be change into shared ptr, because x is then passed to other Python methods.

This issue can be solved in concert with #556 which may introduce interface changes too.