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/Serialize.cpp

@@ -3,7 +3,7 @@
 //  BornAgain: simulate and fit reflection and scattering
 //
 //! @file      ff/Serialize.cpp
-//! @brief     Implements serializing and deserializing of topologies.
+//! @brief     Implements serialization and deserialization for polyhedra.
 //!
 //! @homepage  https://jugit.fz-juelich.de/mlz/libformfactor
 //! @license   GNU General Public License v3 or higher (see COPYING)