void checkin(Patron& p, Book& b)
{   /*
     this function receives by reference a patron and book instance
     changes relevant checkout status
     returns nothing
     */
    b.setCheckOutStatus(0);//sets book checkout status
    p.setBorrowedIn(b);//resets array to not include returned book
    p.setNumberBorrowedIn();//reduces borrowed number by 1
}