rework pointwise operations

nifty6/domains/rg_space.py

nifty6/field.py

nifty6/library/correlated_fields.py

nifty6/library/dynamic_operator.py

nifty6/library/special_distributions.py

nifty6/linearization.py

nifty6/multi_field.py

nifty6/operators/energy_operators.py

nifty6/operators/operator.py

nifty6/sugar.py

test/test_energy_gradients.py

test/test_field.py

test/test_gaussian_energy.py

test/test_linearization.py

test/test_multi_field.py

test/test_operators/test_convolution_operators.py

test/test_operators/test_correlated_fields.py

test/test_operators/test_interpolated.py

test/test_operators/test_jacobian.py

test/test_operators/test_partial_multifield_insert.py