Writing objects with kOverwrite flag instead of kWriteDelete

[lobis]

Changed all the occurences of kWriteDelete for kOverwrite.

This avoids having random numbers after keys in the root file, such as:

root [1] .ls
TFile**         /tmp/tmp.NR4gQ6jbwZ/source/packages/restG4/examples/04.MuonScan/muons.root
 TFile*         /tmp/tmp.NR4gQ6jbwZ/source/packages/restG4/examples/04.MuonScan/muons.root
  KEY: TRestAnalysisTree        AnalysisTree;4  AnalysisTree
  KEY: TTree    EventTree;4     TRestGeant4EventTree
  KEY: TRestRun CosmicMuonRun;4 A basic muon test with 2 active volumes
  KEY: TRestGeant4Metadata      restG4 run;3    MuonsFromWall
  KEY: TRestGeant4PhysicsLists  default;3       First physics list implementation.
  KEY: TGeoManager      Geometry;1      Geometry imported from GDML

Not sure if there was a reason to use kWriteDelete in the first place @nkx111 @jgalan .

[jgalan]

Not sure if there was a reason behind.