schemas/vegbien.sql: specimenreplicate: Fixing specimenreplicate->taxonoccurrence mapping bug where taxonoccurrence_id is no longer used as an fkey because it's instead a pkey inherited from taxonoccurrence, by instead adding an fkey to plantobservation for direct vouchers. Also, it makes more sense for a specimenreplicate to directly voucher the plant it came from rather than that plant's taxonoccurrence, because a direct voucher is a closer relationship to the plant.