office: create policy for what to do about unknown members
previously, encountering an unknown member meant that any parser of this type would abort. now, the user can set parser.unknown_member_policy to either 'omit' or 'keep' if they don't want the current action of 'abort' note that this causes pylint to complain about branching depth for remove_all() because of the nuanced error-handling. I've disabled this check.
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