Add pythia8 XeXe HI parameters to speed up init

Author: preghenella

examples/pythia8/pythia8_XeXe.cfg

@@ -7,9 +7,9 @@ Beams:idB 1000541290		# Xe
 Beams:eCM 5860. 		# GeV
 
 ### heavy-ion settings (valid for Xe-Xe 5860 only)
-#HeavyIon:SigFitNGen = 0
-#HeavyIon:SigFitDefPar = 
-#HeavyIon:bWidth = 
+HeavyIon:SigFitNGen = 0
+HeavyIon:SigFitDefPar = 13.85,1.82,0.22,0.0,0.0,0.0,0.0,0.0
+#HeavyIon:bWidth = 12.53
 
 ### processes (apparently not to be defined)