.present returns std::nullopt if the optional argument is not given by the
user -- as long as a .default_value is not defined. With a .default_value,
.present cannot be used to determine if a value is user-provided or the
default.
.is_used fills that role and only returns true if the argument was passed
by the user.
Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>
