Connected the Cython code for explicit smoothing with the SmoothingOperator.

nifty/operators/smoothing_operator/smooth_util.pyx

nifty/operators/smoothing_operator/smoothing_operator.py

nifty/spaces/gl_space/gl_space.py

nifty/spaces/hp_space/hp_space.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/power_space/power_space.py

nifty/spaces/rg_space/rg_space.py

nifty/spaces/space/space.py

setup.py