Merge branch 'simplify_for_const' into 'NIFTy_5'

nifty5/field.py

nifty5/multi_field.py

nifty5/operators/chain_operator.py

nifty5/operators/operator.py

nifty5/operators/scaling_operator.py

nifty5/operators/simple_linear_operators.py

nifty5/operators/sum_operator.py

test/test_operators/test_simplification.py