nc_retain_youth: Simulate retention curves for youths
In southwick-associates/lifetime: Provide functions for lifetime pricing

Based on the assumption that youths won't generate revenue until age 16 (default). Any youth years are to be ignored for revenue purposes (other than wsfr lifetime dollars). Retention curves are unknown for youth buyers. The simple assumption is that their curves will match 16-year-olds (at age 16). This likely overestimates their future participation.

1	nc_retain_youth(retain_all, youth_ages = 0:15)

`retain_all`	full set of retention curve results produced by nc_retain_all()
`youth_ages`	if not NULL, assumes for youths that the fund is able to compound until adulthood (when the agency will begin drawing revenue). See `nc_price_lifetime_youth` for details.

Other wrapper functions for NC results: nc_break_even_yrs, nc_break_even, nc_price_lifetime_youth, nc_retain, nc_revenue