Reconsider `grace_period` naming in `TreatmentEffect`

[JabobKrauskopf]

Quote from Doc String:
“The grace period is the period after treatment that is not considered in the estimation.”

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Definitions from papers:
“Periods during which treatment interruption is allowed” - https://arxiv.org/pdf/2212.11398
“Waiting periods (periods during which a continued effect of the treatment is assumed after the daily supply has been exhausted)” - https://academic.oup.com/aje/article-abstract/194/3/802/7723772