Restructure operator tests

ChangeLog.md

demos/polynomial_fit.py

src/extra.py

test/test_energy_gradients.py

test/test_gaussian_energy.py

test/test_multi_field.py

test/test_operators/test_adjoint.py

test/test_operators/test_conjugation_operator.py

test/test_operators/test_contraction_operator.py

test/test_operators/test_correlated_fields.py

test/test_operators/test_einsum.py

test/test_operators/test_integration.py

test/test_operators/test_jacobian.py

test/test_operators/test_nft.py

test/test_operators/test_normal_operators.py

test/test_operators/test_partial_multifield_insert.py

test/test_operators/test_representation.py

test/test_operators/test_selection_operators.py

test/test_operators/test_vdot_operator.py