Fix variable assignment mypy error localization

[cariluo]

This PR addresses issue #283, which is about a mypy error in localization.

• Remove ignore[assignment] flags
• rename variables
• Add helper function to decrease duplicated lines of code

[cariluo]

I have renamed variables to follow the existing naming convention in the file of adding unr to the names of variables. That being said, it might be more descriptive to separate out alpha and beta? I'm imagining something like

alpha_S_ = multi_dot((C_[0].T, self.S, C_[0]))
beta_S_  = multi_dot((C_[1].T, self.S, C_[1]))

instead of

unr_S_ = [multi_dot((C_[s].T, self.S, C_[s])) for s in [0, 1]]

[mscho527]

perhaps _a and _b like lmo_coeff?

[mscho527]

it might be possible to remove # type: ignore[attr-defined] after the renaming

[cariluo]

it might be possible to remove # type: ignore[attr-defined] after the renaming

If I remove # type: ignore[attr-defined] I get

src/quemb/molbe/mbe.py:1431: error: "BE" has no attribute "C_a"  [attr-defined]
src/quemb/molbe/mbe.py:1434: error: "BE" has no attribute "C_b"  [attr-defined]
src/quemb/molbe/mbe.py:1437: error: "BE" has no attribute "W_unr"  [attr-defined]
src/quemb/molbe/mbe.py:1437: error: "BE" has no attribute "C_a"  [attr-defined]
src/quemb/molbe/mbe.py:1438: error: "BE" has no attribute "W_unr"  [attr-defined]
src/quemb/molbe/mbe.py:1438: error: "BE" has no attribute "C_b"  [attr-defined]

Would you prefer that I add these attributes to BE __init__() or leave the # type: ignore[attr-defined] flags?