Consider adding helper methods to AnalysisPeriod enum

[mikewilli]

Created from PR comment:

Not sure if there's an obvious solution, but we have a lot of locally defined lists each with a subset of AnalysisPeriod's values. Would be nice if we could do something like AnalysisPeriod.get_pre_enrollment_periods() or AnalysisPeriod.get_all(), etc. so that it's clear what the lists mean, but I don't think that's is a normal pattern for enums.

Nothing to fix necessarily, just hoping this might lead to a better idea than what I've come up with so far.

Originally posted by @mikewilli in #2019 (comment)

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