Add ability to set truncation threshold for SparsePauliOp's simplify method

[BryceFuller]

SparsePauliOp.simplify() has an implicit threshold for truncating small coefficients, the truncation of these coefficients is not tracked by OBP's error budgeting, so these parameters need to be exposed to users as they introduce un-accounted for sources of error.

The solution implemented here is to add the absolute and relative tolerance values atol, rtol, to the OperatorBudget data structure. These values are then passed along to SparsePauliOp.simplify when it gets called.

[coveralls]

Pull Request Test Coverage Report for Build 13394792748

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 13182974064:	0.0%
Covered Lines:	433
Relevant Lines:	433

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💛 - Coveralls

[mrossinek]

I agree with the proposal 👍

Can you please do the following:

• add docstrings to the new attributes of OperatorBudget
• run the style linter: tox -e style
• update the expected output of the unittests
• add a release note describing the new feature

That should fix the CI on all accounts (I think). Ideally, we could also add some unittests to assert that changing atol and rtol actually has an effect on the result, but I am willing to forego that for this PR and address that later as I am confident in the code as is.