[Geom] Add auxiliary user-data export in TGDMLWrite

This adds the option of exporting auxiliary of TGeoVolumes in TGDMLWrite
Fixes #20356

Author: KoljaFrahm

geom/gdml/src/TGDMLWrite.cxx

@@ -156,7 +156,10 @@ See that function for details.
 #include "TGeoElement.h"
 #include "TGeoShape.h"
 #include "TGeoCompositeShape.h"
+#include "TGeoExtension.h"
 #include "TGeoOpticalSurface.h"
+#include "TMap.h"
+#include "TObjString.h"
 #include <cstdlib>
 #include <string>
 #include <map>
@@ -759,6 +762,41 @@ void TGDMLWrite::ExtractVolumes(TGeoNode *node)
       fGdmlE->AddChild(volumeN, childN);
    }
 
+   // export auxiliary user-data if present (TMap of TObjString->TObjString)
+   {
+      TGeoRCExtension *rcext = (TGeoRCExtension *)volume->GrabUserExtension();
+      if (rcext) {
+         TMap *auxmap = nullptr;
+         TObject *userObj = rcext->GetUserObject();
+         if (userObj && userObj->InheritsFrom("TMap")) {
+            TMap *auxmap = (TMap *)userObj;
+            TIterator *it = auxmap->MakeIterator();
+            TObject *k = nullptr;
+            while (k = it->Next()) {
+               TObject *valobj = auxmap->GetValue(k);
+               if (!valobj || !k->InheritsFrom("TObjString") || !valobj->InheritsFrom("TObjString"))
+                  continue;
+               TObjString *key = (TObjString *)k;
+               TObjString *val = (TObjString *)valobj;
+               TString auxtype = key->GetString();
+               TString auxvalue = val->GetString();
+               TString auxunit = "";
+               Int_t pos = auxvalue.Index('*');
+               if (pos >= 0) {
+                  auxunit = auxvalue(pos + 1, auxvalue.Length());
+                  auxvalue = auxvalue(0, pos);
+               }
+               XMLNodePointer_t auxN = fGdmlE->NewChild(nullptr, nullptr, "auxiliary", nullptr);
+               fGdmlE->NewAttr(auxN, nullptr, "auxtype", auxtype.Data());
+               fGdmlE->NewAttr(auxN, nullptr, "auxvalue", auxvalue.Data());
+               if (!auxunit.IsNull())
+                  fGdmlE->NewAttr(auxN, nullptr, "auxunit", auxunit.Data());
+               fGdmlE->AddChild(volumeN, auxN);
+            }
+         }
+      }
+   }
+
    fVolCnt++;
    // add volume/assembly node into the <structure> node
    fGdmlE->AddChild(fStructureNode, volumeN);