Merge branch 'pa_tweaks4' into 'NIFTy_8'

src/linearization.py

src/operators/chain_operator.py

src/operators/diagonal_operator.py

src/operators/energy_operators.py

src/operators/linear_operator.py

src/operators/operator.py

src/operators/scaling_operator.py

test/test_operators/test_diagonal_operator.py