Return a set of 2D ``(x, y)`` coordinates from a linear index.

def index_to_coordinates(self, index):
        """ Return a set of 2D ``(x, y)`` coordinates from a linear index. """
        self.validate_index(index)
        x = int(index % self.length)
        y = int((index - x) / self.length)
        return x, y