Design, code, and test a program that includes a function of type double called yongl that accepts two double arguments $($you must write the yongl function$).$ When called, your function will add the two arguments then divide the sum by $2.$ It will then return that result. Your main function should call yongl then display the value returned by the function. Test your function thoroughly then submit output using the arguments $16\mathrm{.}1$ and $1\mathrm{.}16.$ Remember that a structured function must have have only one return statement. Your program must be structured, commented, professional in appearance, and use only symbolic constants as necessary. $(80$ points$)$