Merge branch 'master' into byebye_cython

demos/wiener_filter.py

demos/wiener_filter_harmonic.py

demos/wiener_filter_unit.py

nifty/operators/__init__.py

nifty/operators/composed_operator/composed_operator.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/endomorphic_operator/endomorphic_operator.py

...rs/invertible_operator_mixin/invertible_operator_mixin.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/projection_operator/projection_operator.py

nifty/operators/propagator_operator/__init__.py

...ators/propagator_operator/harmonic_propagator_operator.py

nifty/operators/propagator_operator/propagator_operator.py

nifty/operators/response_operator/__init__.py

nifty/operators/response_operator/response_operator.py

nifty/operators/smoothing_operator/smoothing_operator.py

nifty/spaces/rg_space/rg_space.py

nifty/sugar.py

test/test_init.py

test/test_operators/test_diagonal_operator.py