CbmEvent: GetIndex() function re-implimentation and constant qualifiers for getters

• setting const qualifier to the GetIndex and the GetStsTrackIndex functions
• re-implimenting the GetIndex function to make it const qualified and reduce the number of access calls (from 3 to 1)

core/data/CbmEvent.cxx

@@ -39,12 +39,17 @@ void CbmEvent::ClearData(ECbmDataType type) { fIndexMap[type].clear(); }
 
 
 // -----   Get a data index   ----------------------------------------------
-uint32_t CbmEvent::GetIndex(ECbmDataType type, uint32_t iData)
+uint32_t CbmEvent::GetIndex(ECbmDataType type, uint32_t iData) const
 {
-
-  if (fIndexMap.find(type) == fIndexMap.end()) return -1;
-  if (fIndexMap[type].size() <= iData) return -2;
-  return fIndexMap.at(type)[iData];
+  auto it = fIndexMap.find(type);
+  if (it == fIndexMap.end()) {
+    return -1;
+  }
+  const auto& indices = it->second;
+  if (indices.size() <= iData) {
+    return -2;
+  }
+  return indices[iData];
 }
 // -------------------------------------------------------------------------