Move accessor functions to separate source file
Rename accessor functions to be consistent

    dt_day_of_year(dt_t dt)    => dt_doy(dt_t dt)
    dt_day_of_quarter(dt_t dt) => dt_doq(dt_t dt)
    dt_day_of_month(dt_t dt)   => dt_dom(dt_t dt)
    dt_week_of_year(dt_t dt)   => dt_woy(dt_t dt)