Extend the MP2InitialPoint computation to unrestricted spin cases

[declanmillar]

What should we add?

The MP2InitialPoint computation, which was introduced in PR #590, seems to only support use cases where the alpha and beta spin MOs are identical. I.e., came out of a restricted spin calculation. Extend the MP2InitialPoint computation to support unrestricted spin cases.

Suggestion

@mrossinek's suggestion for the future when extending this to unrestricted spin cases:

-    i = excitation[0][0] % num_molecular_orbitals
+    a_or_b_i, i = divmod(excitation[0][0], num_molecular_orbitals)

a_or_b_i will then be either 0 or 1 indicating which set of 2-body terms to use.