all ingredients for critical filtering, nothing tested

nifty/field.py

nifty/library/energy_library/critical_power_energy.py

nifty/library/operator_library/__init__.py

nifty/library/operator_library/critical_power_curvature.py

nifty/library/operator_library/laplace_operator.py

nifty/library/operator_library/smoothness_operator.py