Complete Point2D::distanceFromOrigin, a member function that computes how far the point (x, y) is from the
origin, using the standard distance formula sqrt(x^2 + y^2).
This is the smallest possible class with real behavior: two public member variables, and one member function that
reads them without needing them passed in as parameters – x and y are already in scope inside any of
Point2D’s own member functions, because they belong to whichever object the function was called through.