Triangle setters should null out the cached center and normal #957

com.jme3.math.Triangle generates a center vector and a normal vector and caches them. However, altering the triangle's vertices (using set(int,Vector3f), set(int, float, float, float), set1(Vector3f), set2(Vector3f), set3(Vector3f) or set(Vector3f, Vector3f, Vector3f)) does not update or invalidate the cached vectors. As a consequence, getCenter() and getNormal() may return incorrect values.