De/Serialization for wavefront object files

Adds to export/import topology as wavefront object files. This enables users to view/edit/use topology inside 3rd party 3D editors

ff/Polyhedron.h

@@ -36,8 +36,8 @@ public:
     void assert_platonic() const;
     double volume() const;
     double radius() const;
-    const PolyhedralTopology& topology() { return m_topology; };
-    const std::vector<R3>& vertices() { return m_vertices; };
+    const PolyhedralTopology& topology() const { return m_topology; };
+    const std::vector<R3>& vertices() const { return m_vertices; };
     complex_t formfactor(const C3& q) const;
 
 private: