Add(NumberTheory/ModularForms/Identities): periods of modular forms.

[CBirkbeck]

trafficstars

Add some basic results about ModularGroup.T relating to slash invariant forms of level Gamma(N) and moving elements into verticalStrips. Needed for #12456

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[Ruben-VandeVelde]

maintainer merge

(preferably a maintainer who knows about modular forms)

[github-actions[bot]]

🚀 Pull request has been placed on the maintainer queue by Ruben-VandeVelde.

[jcommelin]

Thanks :tada:

bors merge

[mathlib-bors[bot]]

Pull request successfully merged into master.

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