merge from master

demos/wiener_filter_easy.py

nifty/config/nifty_config.py

nifty/field.py

nifty/operators/fft_operator/fft_operator.py

nifty/operators/smoothing_operator/fft_smoothing_operator.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/rg_space/rg_space.py

nifty/spaces/space/space.py

test/test_spaces/test_lm_space.py

test/test_spaces/test_power_space.py

test/test_spaces/test_rg_space.py