more documentation

nifty4/operators/adjoint_operator.py

nifty4/operators/chain_operator.py

nifty4/operators/diagonal_operator.py

nifty4/operators/dof_distributor.py

nifty4/operators/endomorphic_operator.py

nifty4/operators/fft_operator.py

nifty4/operators/geometry_remover.py

nifty4/operators/harmonic_transform_operator.py

nifty4/operators/inverse_operator.py

nifty4/operators/inversion_enabler.py

nifty4/operators/laplace_operator.py

nifty4/operators/power_distributor.py

nifty4/operators/scaling_operator.py

nifty4/operators/sum_operator.py