reduce direct usage of Field constructor

demos/critical_filtering.py

demos/nonlinear_critical_filter.py

demos/paper_demos/cartesian_wiener_filter.py

demos/paper_demos/wiener_filter.py

demos/wiener_filter_via_hamiltonian.py

nifty4/domains/lm_space.py

nifty4/field.py

nifty4/operators/dof_distributor.py

test/test_field.py

test/test_operators/test_adjoint.py

test/test_operators/test_smoothing_operator.py