feat: permit the disabling of Sz conservation

[mrossinek]

This PR allows an end-user to disable the Sz conservation in a TeNPy-based dynamic MPF calculation.

Previously, we did not support this as stated in #36.
TeNPy's XXZChain2 class actually hard-codes the Sz conservation, contrary to my statement in #36.
The user can do so manually with a custom subclass until TeNPy supports this natively.

With this PR, the user has full flexibility over enabling/disabling Sz conservation.

Closes #36.

[coveralls]

Pull Request Test Coverage Report for Build 12635192604

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file 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 12634786731:	0.0%
Covered Lines:	581
Relevant Lines:	581

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💛 - Coveralls