move arithmetic functions working on Fields to sugar

nifty4/__init__.py

nifty4/domains/lm_space.py

nifty4/domains/rg_space.py

nifty4/field.py

nifty4/library/noise_energy.py

nifty4/library/nonlinear_power_energy.py

nifty4/library/nonlinearities.py

nifty4/library/poisson_energy.py

nifty4/multi/multi_domain.py

nifty4/sugar.py