merge master

nifty/operators/composed_operator/composed_operator.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/endomorphic_operator/endomorphic_operator.py

nifty/operators/projection_operator/projection_operator.py

...ators/propagator_operator/harmonic_propagator_operator.py

nifty/operators/propagator_operator/propagator_operator.py

nifty/operators/response_operator/response_operator.py

nifty/operators/smoothing_operator/smoothing_operator.py

test/test_operators/test_composed_operator.py

test/test_operators/test_response_operator.py

test/test_operators/test_smoothing_operator.py