Fixed expectation_computational_basis_state

[S-Erik]

Trying to fix expectation_computational_basis_state. The implemented fix was proposed in issue #515.
I also added a test that covers the new exception that is raised when the provided operator is normal ordered.

Changes I made:

1. Checking for occupied orbitals in the inner j-loop:

            for j in range(i + 1, len(occupied_orbitals)):
                if occupied_orbitals[j]:
                    expectation_value -= operator.terms.get(((j, 1), (i, 1), (j, 0), (i, 0)), 0.0)

2. Making more clear how a list of occupations has to be provided:

            computational basis state (if scipy.sparse vector), or list of
            zeros and ones for occupied and unoccupied orbitals, respectively.

3. Adding a test that varifies that the new ValueError exception is raised if the provided operator is not normal-ordered:

    def test_expectation_bad_operator_order(self):
        operator = (
            FermionOperator('2^ 2', 1.9)
            + FermionOperator('2^ 1')
            + FermionOperator('2^ 1 2 1^', -1.7)
        )
        state = [0, 1, 1]

        with self.assertRaises(ValueError):
            expectation_computational_basis_state(operator, state)

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[mhucka]

@S-Erik Thank you for this work. Quick request: could you comment on issue #515? Any comment will do, like mentioning you did this PR – it's just for GitHub to allow me to assign you to that issue, to indicate it's being worked on.

[mhucka]

Regarding the current failures in CI: I'm waiting until I can merge #1204 and then will run CI on this PR again.

[mhucka]

LGTM

[mhucka]

@S-Erik There remained some trivial code formatting issues. In order to expedite things, I took the liberty of pushing a fix to your branch – I hope you don't mind. The result worked and this PR is now in the merge queue. I hope the push to your branch does not cause you any problems on your end.

Thanks again for this contribution.