- added "material" support to csg core:
 * this is just to lay out the ground work for future evolution (like defining various material properties for 3d printing: springiness, conductivity etc)
 * for now , "color" is the only property of the material classes
 * this is also in order to have a more OOP api for colors etc
 * need a bit of unit tests, but already "manually" tested (see exampledump file)