standardize_age_groups() does too much

[gadenbuie]

standardize_age_groups() does a lot of work to try to resolve age groups. This is good for non-standard age groups, and handles cases like merging "5 - 7" and "8 - 9" into the standard "5 - 9".

But if the age groups perfectly match those in fcds_const("age_groups"), this is far too much work and also results in a scary "NA introduced by coercion" warnings.