Merge branch 'master' into byebye_fixed_point_voodoo

demos/wiener_filter_easy.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py

nifty/field.py

nifty/minimization/line_searching/line_search.py

...y/operators/fft_operator/transformations/rg_transforms.py

...rs/invertible_operator_mixin/invertible_operator_mixin.py

nifty/operators/smoothing_operator/fft_smoothing_operator.py

nifty/plotting/plotter/healpix_plotter.py

nifty/probing/prober/prober.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/power_space/power_space.py

test/test_operators/test_fft_operator.py

test/test_serialization.py