Add a length multiplier option for XDG meshes.

include/openmc/xdg.h

@@ -24,7 +24,8 @@ class XDGMesh : public UnstructuredMesh {
   XDGMesh(pugi::xml_node node);
   XDGMesh(hid_t group);
   XDGMesh(const std::string& filename, double length_multiplier = 1.0);
-  XDGMesh(std::shared_ptr<xdg::XDG> external_xdg);
+  XDGMesh(
+    std::shared_ptr<xdg::XDG> external_xdg, double length_multiplier = 1.0);
 
   static const std::string mesh_interface;
 

src/xdg.cpp

@@ -83,10 +83,12 @@ XDGMesh::XDGMesh(const std::string& filename, double length_multiplier)
   initialize();
 }
 
-XDGMesh::XDGMesh(std::shared_ptr<xdg::XDG> external_xdg)
+XDGMesh::XDGMesh(
+  std::shared_ptr<xdg::XDG> external_xdg, double length_multiplier)
 {
   xdg_ = external_xdg;
   filename_ = "unknown (external file)";
+  set_length_multiplier(length_multiplier);
   initialize();
 }