parse
Parses an ISO-8601 duration string into a BillingPeriod.
Accepts the date-portion forms Play emits (PnYnMnWnD, any subset, in that order). The M (months) designator must be uppercase — lowercase m means minutes in ISO-8601 — but P, Y, W and D are accepted in either case. Time components (PT1H) are not used by Play billing periods and are rejected.
Return
the parsed period, or null if iso is null, blank, or not a well-formed date-only ISO-8601 duration.
Parameters
iso
the raw period string, e.g. "P1M". May be null.