Refactor all math into a new kimath library

* Split up the thirdparty code into the thirdparty folder (#3637)
* Create a new kimath static library containing all the math functions

This is part of cleaning the build system for #1906.

common/plotters/PS_plotter.cpp

@@ -36,6 +36,7 @@
 #include <macros.h>
 #include <kicad_string.h>
 #include <convert_basic_shapes_to_polygon.h>
+#include <math/util.h>      // for KiROUND
 
 /* Forward declaration of the font width metrics
    (yes extern! this is the way to forward declare variables */