MuonReadoutGeometryR4: Speed up MdtTubeLayer::getTubeNode().

This was using getChildVol. However, that keeps track of the transform to each child, even though that is not actually used. This is slow, especially in debug builds where Eigen operations tend to be expensive.

Instead, use GeoVisitVolumes to iterate over children without calculating the transforms.

MuonSpectrometer/MuonPhaseII/MuonDetDescr/MuonReadoutGeometryR4/src/MdtTubeLayer.cxx

@@ -7,6 +7,7 @@
 
 #include <GeoModelHelpers/TransformSorter.h>
 #include <GeoModelHelpers/GeoPhysVolSorter.h>
+#include <GeoModelUtilities/GeoVisitVolumes.h>
 
 namespace MuonGMR4{
 
@@ -38,13 +39,46 @@ MdtTubeLayer::MdtTubeLayer(const PVConstLink layer,
 const Amg::Transform3D& MdtTubeLayer::layerTransform() const {
     return m_layTrf->getDefTransform();
 }
+namespace {
+// Helper to find the Nth child volume, without keeping track
+// of the transform.
+class GeoGetChild : public IGeoVisitVolumesNoXformAction
+{
+public:
+  GeoGetChild (size_t n) : m_n (n) {}
+  virtual void operator() (GeoNodeAction& action,
+                           int /*id*/,
+                           const std::string& /*name*/,
+                           const GeoVPhysVol* volume) override
+  {
+    if (m_n == 0) {
+      m_node = volume;
+      action.terminate();
+    }
+    else {
+      --m_n;
+    }
+  }
+
+  size_t m_n;
+  const GeoVPhysVol* m_node = nullptr;
+};
+} // anonymous namespace
 PVConstLink MdtTubeLayer::getTubeNode(unsigned int tube) const {
      if (tube >= nTubes()) {
         std::stringstream except{};
         except<<__FILE__<<":"<<__LINE__<<" "<<m_layerNode->getLogVol()->getName()<<" has only "<<nTubes()<<" tubes. But "<<tube<<" is requested. Please check.";
         throw std::out_of_range(except.str());
     }
-    return m_layerNode->getChildVol(tube);
+
+    // Don't use getChildVol.  Under the hood, that keeps track of the
+    // transform to each child, even though that is not actually used.
+    // This is slow, especially in debug builds where Eigen operations
+    // tend to be expensive.
+    GeoGetChild a (tube);
+    GeoVisitVolumes visitor (a);
+    m_layerNode->exec (&visitor);
+    return a.m_node;
 }
 const Amg::Transform3D MdtTubeLayer::tubeTransform(const unsigned int tube) const {    
     if (tube >= nTubes()) {