restructure iteration controllers to match those in nightly branch

demos/critical_filtering.py

demos/log_normal_wiener_filter.py

demos/paper_demos/cartesian_wiener_filter.py

demos/paper_demos/wiener_filter.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py

nifty/minimization/__init__.py

nifty/minimization/iteration_controlling/__init__.py

...ization/iteration_controlling/gradient_norm_controller.py

...inimization/iteration_controlling/iteration_controller.py

test/test_minimization/test_minimizers.py