avoid assert statements; simplify interface to GradientNormController

demos/critical_filtering.py

demos/log_normal_wiener_filter.py

demos/nonlinear_critical_filter.py

demos/paper_demos/cartesian_wiener_filter.py

demos/paper_demos/wiener_filter.py

demos/wiener_filter_easy.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py

nifty4/minimization/gradient_norm_controller.py

nifty4/minimization/line_search_strong_wolfe.py

nifty4/operators/harmonic_transform_operator.py

nifty4/spaces/power_space.py

test/test_minimization/test_minimizers.py