controller: Go back to `int` vs `size_t` for level iterator.

This allows the following to work properly:

    for (auto leviter = this->finest() - 1; leviter >= this->coarsest(); --leviter) {

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge branch 'feature/mpi-pf' of github.com:memmett/PFASST into feature/mpi-pf