Rearranged operators from library. Refactored wiener_filter operators.

nifty/library/__init__.py

nifty/library/critical_filter/__init__.py

nifty/library/critical_filter/critical_power_curvature.py

nifty/library/critical_filter/critical_power_energy.py

nifty/library/wiener_filter/__init__.py

nifty/library/wiener_filter/wiener_filter_curvature.py

nifty/library/wiener_filter/wiener_filter_energy.py

nifty/operators/__init__.py

nifty/operators/laplace_operator/laplace_operator.py

nifty/operators/propagator_operator/__init__.py

...ators/propagator_operator/harmonic_propagator_operator.py

nifty/operators/propagator_operator/propagator_operator.py

nifty/operators/smoothness_operator/__init__.py

nifty/operators/smoothness_operator/smoothness_operator.py

nifty/sugar.py