Modernize xgen test

Use ThOr functors as much as possible in example. Add a description so users can use the example to test decay files. The goal is to advertise this at https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/blob/master/CONTRIBUTING.md .

More functors can be added as required by the simulation project. Please comment here.

Related to but not (yet) dependent on Analysis!846 (merged)

DaVinciExamples/python/DaVinciExamples/tupling/option_davinci_tupling_from_xgen.py

 ###############################################################################
-# (c) Copyright 2021-2022 CERN for the benefit of the LHCb Collaboration      #
+# (c) Copyright 2022 CERN for the benefit of the LHCb Collaboration           #
 #                                                                             #
 # This software is distributed under the terms of the GNU General Public      #
 # Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
@@ -10,10 +10,19 @@
 ###############################################################################
 """
 Read and process a .xgen file with the new DaVinci configuration.
+
+This is run as 
+./run davinci run-mc --inputfiledb test_read_xgen Phys/DaVinci/options/DaVinciDB-Example.yaml --user_algorithms DaVinciExamples/python/DaVinciExamples/tupling/option_davinci_tupling_from_xgen:main
+
+To test with another file, create a yaml file and do 
+
+./run davinci run-mc --inputfiledb fileKey file.yaml --user_algorithms DaVinciExamples/python/DaVinciExamples/tupling/option_davinci_tupling_from_xgen:main
+
 """
 from FunTuple import FunctorCollection, FunTuple_MCParticles as FuntupleMC
 from PyConf.Algorithms import PrintMCTree
 from PyConf.dataflow import force_location
+import Functors as F
 
 #FunTuple: define fields (branches)
 fields = {
@@ -26,19 +35,15 @@ fields = {
 
 #FunTuple: define variables for the B meson
 variables_B = FunctorCollection({
-    'ETA': 'MCETA',
-    'PHI': 'MCPHI',
-    'PT': 'MCPT',
-    'E': 'MCE',
+    'ETA': F.ETA,
+    'PHI': F.PHI,
+    'VX': 'MCVFASPF(MCVX)',  #LoKi
+    'VY': 'MCVFASPF(MCVY)',  #LoKi
+    'VZ': 'MCVFASPF(MCVZ)',  #LoKi
 })
 
 #FunTuple: define common variables
-variables_all = FunctorCollection({
-    'P': 'MCP',
-    'PX': 'MCPX',
-    'PY': 'MCPY',
-    'PZ': 'MCPZ',
-})
+variables_all = FunctorCollection({'PT': F.PT, 'P': F.FOURMOMENTUM})
 
 #FunTuple: associate functor collections to field (branch) name
 variables = {