make Fields and MultiFields immutable

nifty5/domain_tuple.py

nifty5/domains/domain.py

nifty5/domains/log_rg_space.py

nifty5/domains/rg_space.py

nifty5/extra/operator_tests.py

nifty5/field.py

nifty5/minimization/conjugate_gradient.py

nifty5/minimization/energy.py

nifty5/minimization/energy_sum.py

nifty5/minimization/line_energy.py

nifty5/minimization/quadratic_energy.py

nifty5/minimization/scipy_minimizer.py

nifty5/minimization/vl_bfgs.py

nifty5/multi/multi_field.py

nifty5/operators/diagonal_operator.py

nifty5/operators/dof_distributor.py

nifty5/operators/inversion_enabler.py

nifty5/operators/scaling_operator.py

nifty5/operators/selection_operator.py

nifty5/operators/symmetrizing_operator.py