A few small improvements

[garrison]

I've had this branch sitting locally for a while; I think it's time to contribute it.

• Use appropriate exceptions instead of AttributeError
• Use tuple unpacking rather than indexing (will catch errors better)
• raise NotImplementedError for methods not implemented
• Remove double variable assignment in docs/conf.py

[coveralls]

Pull Request Test Coverage Report for Build 4620590636

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 6 (33.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on small-improvements at 84.714%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
circuit_knitting_toolbox/entanglement_forging/entanglement_forging_ground_state_solver.py	2	6	33.33%

Totals
Change from base Build 4620488455:	84.7%
Covered Lines:	1524
Relevant Lines:	1799

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💛 - Coveralls

[caleb-johnson]

Nice, thanks