more locking and diagnostics

demos/nonlinear_critical_filter.py

nifty4/domains/power_space.py

nifty4/domains/rg_space.py

nifty4/library/critical_power_energy.py

nifty4/library/nonlinear_power_energy.py

nifty4/library/nonlinear_wiener_filter_energy.py

nifty4/library/wiener_filter_energy.py

nifty4/minimization/conjugate_gradient.py

nifty4/minimization/energy.py

nifty4/minimization/line_energy.py

nifty4/minimization/quadratic_energy.py

nifty4/minimization/scipy_minimizer.py

nifty4/operators/diagonal_operator.py

test/test_field.py

test/test_operators/test_fft_operator.py