Jet Core pixel cluster splitter

This is a first version of the jetcore cluster splitter (removing most of the debugging code, e.g. MC truth access).

This PR doesn't add anything to reco sequence yet but a customize function is made available for tests (timing, performance, tunining)


Hi again,
I confirm the bug that has been introduced after #5807 was integrated. This PR changed the 'iter0TrackRefsForJets' into 'initialStepTrackRefsForJets'.

@arizzi it would be extremely easy for me if you could update this PR, so that we keep on commenting on this one. A simple s/iter0TrackRefsForJets/initialStepTrackRefsForJets/ inside this customize file will do the trick. Let me know in case you cannot: in that case we need to close this PR and open a new one.

Here is in case the complete patch:

```
diff --git a/RecoTracker/Configuration/python/customizeForClusterSplitting.py b/RecoTracker/Configuration/python/customizeForClusterSplitting.py
index 2b3f794..56d9059 100644
--- a/RecoTracker/Configuration/python/customizeForClusterSplitting.py
+++ b/RecoTracker/Configuration/python/customizeForClusterSplitting.py
@@ -43,8 +43,8 @@ def customizeForClusterSplitting(process):
      process.firstStepPrimaryVerticesPreSplitting = process.firstStepPrimaryVertices.clone()
      process.firstStepPrimaryVerticesPreSplitting.TrackLabel = 'initialStepTracksPreSplitting'

-     process.iter0TrackRefsForJetsPreSplitting = process.iter0TrackRefsForJets.clone()
-     process.iter0TrackRefsForJetsPreSplitting.src = 'initialStepTracksPreSplitting'
+     process.initialStepTrackRefsForJetsPreSplitting = process.initialStepTrackRefsForJets.clone()
+     process.initialStepTrackRefsForJetsPreSplitting.src = 'initialStepTracksPreSplitting'
      process.caloTowerForTrkPreSplitting = process.caloTowerForTrk.clone()
      process.ak4CaloJetsForTrkPreSplitting = process.ak4CaloJetsForTrk.clone()
      process.ak4CaloJetsForTrkPreSplitting.src = 'caloTowerForTrkPreSplitting'
@@ -64,7 +64,7 @@ def customizeForClusterSplitting(process):
                                                     process.initialStepTrackCandidatesPreSplitting +
                                                     process.initialStepTracksPreSplitting +
                                                     process.firstStepPrimaryVerticesPreSplitting +
-                                                    process.iter0TrackRefsForJetsPreSplitting +
+                                                    process.initialStepTrackRefsForJetsPreSplitting +
                                                     process.caloTowerForTrkPreSplitting +
                                                     process.ak4CaloJetsForTrkPreSplitting +
                                                     process.jetsForCoreTrackingPreSplitting +
```
Ciao,
M.