optimization for sparse tools

[Spaceenter]

No description provided.

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[Spaceenter]

@babbush Should I sign the CLA with my personal gmail account?

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In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[coveralls]

Coverage increased (+0.002%) to 99.591% when pulling b5cbc02 on Spaceenter:opt into 6c17093 on quantumlib:develop.

[coveralls]

Coverage decreased (-0.03%) to 99.558% when pulling 729fdb5 on Spaceenter:opt into 6c17093 on quantumlib:develop.