some cosmetic fixes and unrelated tweaks

nifty4/data_objects/distributed_do.py

nifty4/field.py

nifty4/library/nonlinear_wiener_filter_energy.py

nifty4/library/wiener_filter_curvature.py

nifty4/library/wiener_filter_energy.py

nifty4/logger.py

nifty4/minimization/line_search.py

nifty4/minimization/line_search_strong_wolfe.py

nifty4/multi/multi_field.py

nifty4/sugar.py

nifty4/utilities.py

test/test_field.py

test/test_minimization/test_minimizers.py

test/test_operators/test_composed_operator.py

test/test_operators/test_diagonal_operator.py