get_k_lengths_array returns a Field now

nifty/operators/diagonal_operator.py

nifty/operators/fft_smoothing_operator.py

nifty/operators/laplace_operator.py

nifty/spaces/lm_space.py

nifty/spaces/power_space.py

nifty/spaces/rg_space.py

nifty/spaces/space.py

test/test_spaces/test_interface.py

test/test_spaces/test_lm_space.py

test/test_spaces/test_rg_space.py