Updating MM tracking surfaces positions

The position of the MM tracking surfaces is moved to the center of the gap along Z, as it should be. MM digitization is updated accordingly, fixing the calculation of the position of the first hit strip on the readout plane. SDO position is also fixed and put at the center of the gap as well. (adding @pscholer who did the fix)

MuonSpectrometer/MuonDetDescr/MuonReadoutGeometry/src/MMReadoutElement.cxx

@@ -301,14 +301,11 @@ namespace MuonGM {
       // identifier of the first channel
       Identifier id = manager()->mmIdHelper()->channelID(getStationName(),getStationEta(),getStationPhi(),m_ml, layer+1, 1);
 
-      // move the readout plane to the sensitive volume boundary 
-      double shift = 0.5*m_etaDesign[layer].thickness;
-      double rox = ( layer==0 || layer==2) ? shift : -shift;
-      
+      /// keep the tracking surface at the center of the gap
       double sAngle = m_etaDesign[layer].sAngle;
       // need to operate switch x<->z because of GeoTrd definition
       m_surfaceData->m_layerSurfaces.push_back( new Trk::PlaneSurface(*this, id) );
-      m_surfaceData->m_layerTransforms.push_back(absTransform()*m_Xlg[layer]*Amg::Translation3D(rox,0.,0.)*
+      m_surfaceData->m_layerTransforms.push_back(absTransform()*m_Xlg[layer]*
 						 Amg::AngleAxis3D(-90.*CLHEP::deg,Amg::Vector3D(0.,1.,0.)) );
 
       // surface info (center, normal)