OPAG_simple updates

[peterdavjohnson]

For now, can just make sure that input pop and standard are the same where they overlap (so can be either single or 5-year).

Add this check:
stopifnot(OAnew >= OAnow)

Change age concordance:

    # age concordance
    minStAge = min(StAge)
    stopifnot(all(Age[Age >= minStAge] %in% StAge))

Add that lengths of standard pop and ages match:
stopifnot(length(StPop) == length(StAge))