Adds SHOULD_NOT_CALL_PARENT - #455

/datum/proc/thing()
    SHOULD_NOT_CALL_PARENT(TRUE)
    CRASH("This should not be called!")

/datum/a/thing()
    // This will throw an error!
    return ..()

/datum/b/thing()
    // This is fine.
    return TRUE

/datum/b/c/thing()
    // This is ALSO fine, as it is a grandchild
    return ..()

/datum/c/d/thing()
    // This will throw also throw an error, as /datum/c does not implement thing()
    return ..()