[PWGEM] Add MC task for photon efficiency and purity studies

Add photon-mc-task, which fills the histograms needed to derive photon
acceptance, efficiency and purity for PCM and EMCal in MC, each vs pT
and centrality or FT0C multiplicity.

For each selected collision, it fills:
- generated true photons within the configurable eta range
- generated photons in detector acceptance (EMCal/DCal geometry incl.
  the PHOS gap for EMCal, the generator eta range for PCM)
- reconstructed candidates passing the photon cuts
- candidates validated as true photons, using the common MC mother of
  the V0 legs for PCM and the leading MC contribution for EMCal

PCM, EMCal and combined PCM+EMCal running are selected by process
switches.

Author: mhemmer-cern

PWGEM/PhotonMeson/Tasks/CMakeLists.txt

@@ -181,6 +181,11 @@ o2physics_add_dpl_workflow(photon-reso-task
                     PUBLIC_LINK_LIBRARIES O2::Framework O2Physics::AnalysisCore O2::EMCALBase O2::EMCALCalib O2Physics::PWGEMPhotonMesonCore
                     COMPONENT_NAME Analysis)
 
+o2physics_add_dpl_workflow(photon-mc-task
+                    SOURCES photonMcTask.cxx
+                    PUBLIC_LINK_LIBRARIES O2::Framework O2Physics::AnalysisCore O2::EMCALBase O2::EMCALCalib O2Physics::PWGEMPhotonMesonCore
+                    COMPONENT_NAME Analysis)
+
 o2physics_add_dpl_workflow(photonhbt
                     SOURCES photonhbt.cxx
                     PUBLIC_LINK_LIBRARIES O2::Framework O2Physics::AnalysisCore O2Physics::PWGEMPhotonMesonCore