more convenient handling of 'unitary' property

nifty/library/log_normal_wiener_filter_curvature.py

nifty/library/nonlinear_power_curvature.py

nifty/library/response_operators.py

nifty/library/wiener_filter_curvature.py

nifty/operators/composed_operator.py

nifty/operators/direct_smoothing_operator.py

nifty/operators/dof_projection_operator.py

nifty/operators/endomorphic_operator.py

nifty/operators/fft_smoothing_operator.py

nifty/operators/laplace_operator.py

nifty/operators/linear_operator.py

nifty/operators/response_operator.py

nifty/operators/smoothness_operator.py