more fixes

nifty6/minimization/conjugate_gradient.py

nifty6/minimization/descent_minimizers.py

nifty6/minimization/line_search.py

nifty6/minimization/nonlinear_cg.py

nifty6/minimization/quadratic_energy.py

nifty6/minimization/scipy_minimizer.py

nifty6/multi_field.py

test/test_multi_field.py

test/test_operators/test_composed_operator.py

test/test_operators/test_diagonal_operator.py

test/test_operators/test_smoothing_operator.py