Remove debug value.
Remove old commented-out lines.
Add Model::material_names to make up for missing materials hash; use it for AMF.
Return undef from Model::get_material if no material.
Return undef from StringMap::FETCH on missing key.
Update TODO comment.
Add Model::delete_all_materials() and call it from ~Model.
Copy some code from model-xs branch

(fixes for set_material and implementation of duplicate_objects_grid).