Added simple dd4hep example

To run root script:
```
root -b -q scripts/example_dd4hep.cxx+
```

scripts/example_dd4hep.cxx

+#include "DD4hep/Detector.h"
+#include "DDG4/Geant4Data.h"
+#include "DDRec/CellIDPositionConverter.h"
+#include "DDRec/SurfaceManager.h"
+#include "DDRec/Surface.h"
+
+
+#include "fmt/core.h"
+
+
+/** Example using dd4hep
+ */
+void example_dd4hep(const char* fname = "test_tracker_disc.root"){
+
+  using namespace ROOT::Math;
+
+  // -------------------------
+  // Get the DD4hep instance
+  // Load the compact XML file
+  // Initialize the position converter tool
+  dd4hep::Detector& detector = dd4hep::Detector::getInstance();
+  detector.fromCompact("athena.xml");
+  dd4hep::rec::CellIDPositionConverter cellid_converter(detector);
+
+  fmt::print("Detector Types:\n");
+  for(const auto&  dtype : detector.detectorTypes() ) {
+    fmt::print("  {}\n", dtype); 
+  }
+
+  fmt::print("\n");
+  fmt::print("All detector subsystem names:\n");
+  for(const auto&  d : detector.detectors() ) {
+    fmt::print("  {}\n", d.first);
+  }
+
+}