Check for same-age-different-year cases

[bpbond]

As noted in https://github.com/forc-db/ForC_private/issues/13 there are cases where different years (for the same site, treatment, etc.) have the same Age_disturbance. I think this is where the authors were ambiguous about what year the given age applied to. Do we want to 'fix'? Or just add a note for downstream users that this may occur, so they can fix as they deem appropriate?