'Moves any koopa in a shell along the x, y axes and checks for
collisions'

def adjust_shell_position(self):

    for shell in self.shell_group:
        shell.rect.x += shell.x_vel
        self.check_shell_x_collisions(shell)
        shell.rect.y += shell.y_vel
        self.check_shell_y_collisions(shell)
        self.delete_if_off_screen(shell)
