merge from master

demos/critical_filtering.py

nifty/field.py

nifty/library/critical_filter/critical_power_energy.py

...rs/invertible_operator_mixin/invertible_operator_mixin.py

nifty/operators/response_operator/response_operator.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/power_space/__init__.py

nifty/spaces/power_space/power_index_factory.py

nifty/spaces/power_space/power_indices.py

nifty/spaces/power_space/power_space.py

nifty/spaces/rg_space/rg_space.py

nifty/spaces/space/space.py

test/test_operators/test_fft_operator.py

test/test_spaces/test_power_space.py