merge NIFTY_4

demos/Wiener_Filter.ipynb

demos/nonlinear_wiener_filter.py

nifty4/__init__.py

nifty4/extra/energy_tests.py

nifty4/field.py

nifty4/minimization/__init__.py

nifty4/minimization/energy_sum.py

nifty4/minimization/line_energy.py

nifty4/minimization/scipy_minimizer.py

nifty4/minimization/yango.py

nifty4/multi/multi_field.py

nifty4/operators/linear_operator.py

nifty4/sugar.py

test/test_multi_field.py