refact(Polar): change from value to handle class

+af_tools/@Polar/Polar.m

-classdef Polar
+classdef Polar < handle
     % POLAR Airfoil polars and some associated properties.
     %   The Polar class is used to store an airfoil polars (cl, cd, cm values for different angles
     %   of attacks, at different Reynolds and Mach numbers).