comment cleanup

nifty/operators/composed_operator.py

nifty/operators/diagonal_operator.py

nifty/operators/direct_smoothing_operator.py

nifty/operators/endomorphic_operator.py

nifty/operators/fft_operator.py

nifty/operators/fft_smoothing_operator.py

nifty/operators/invertible_operator_mixin.py

nifty/operators/laplace_operator.py

nifty/operators/linear_operator.py

nifty/operators/smoothness_operator.py