Place the space-frame sectors explicitly instead of dividing BBMO

This replaces the phi division of the space-frame mother BBMO by 18 explicit
placements of the sector volume BBCE, so that the sector contents survive the
conversion to Geant4.

- Geant4 divides a polyhedra in phi by its number of sides and ignores the
  requested width and offset, and its cell frame sits at the start of a sector
  where TGeo's sits at the centre.
- Both conversion routes, ROOT's GDML writer and VGM, are affected, so the 18
  FTOS service boxes placed in BBCE were absent from the Geant4 geometry.
- The TGeo geometry is unchanged: same node count, same BBCE shape and content,
  same global placements, no new overlaps.
- With this, the ray-by-ray mass thickness of the two Geant4 geometries agrees
  with TGeo to 9e-8 relative, against 5e-5 before.

https://its.cern.ch/jira/browse/O2-7153

Author: pge98bmo

Detectors/Passive/src/FrameStructure.cxx

@@ -1789,7 +1789,21 @@ void FrameStructure::ConstructGeometry()
   ppgon[9] = ppgon[6];
 
   vmc->Gsvolu("BBMO", "PGON", kAir, ppgon, 10);
-  vmc->Gsdvn("BBCE", "BBMO", 18, 2);
+
+  // The 18 sectors, placed one by one rather than made with a phi division.
+  // Geant4 has no faithful representation of a phi division of a polyhedra: it
+  // divides by the number of sides and ignores the requested width and offset,
+  // so the sector contents end up half a sector away from where TGeo puts them.
+  const int kNSectors = 18;
+  const float kSectorDphi = 360. / kNSectors;
+  TGeoPgon* shBBCE = new TGeoPgon(-kSectorDphi / 2., kSectorDphi, 1, 2);
+  shBBCE->DefineSection(0, -kBBMdz / 2., kBBMRin, kBBMRou);
+  shBBCE->DefineSection(1, kBBMdz / 2., kBBMRin, kBBMRou);
+  TGeoVolume* voBBCE = new TGeoVolume("BBCE", shBBCE, kMedAir);
+  TGeoVolume* voBBMO = gGeoManager->GetVolume("BBMO");
+  for (i = 0; i < kNSectors; i++) {
+    voBBMO->AddNode(voBBCE, i + 1, new TGeoRotation("", (i + 0.5) * kSectorDphi, 0., 0.));
+  }
 
   // CBL ////////////////////////////////////////////////////////
   //