PWGLF: move free-floating Preslice<> into task

Author: mario-krueger

PWGLF/Tasks/spectraCharged.cxx

@@ -84,6 +84,7 @@ struct chargedSpectra {
   using CollisionTableMC = soa::SmallGroups<soa::Join<aod::McCollisionLabels, aod::Collisions, aod::EvSels>>;
   using TrackTableMC = soa::Join<aod::Tracks, aod::McTrackLabels, aod::TrackSelection>;
   using ParticleTableMC = aod::McParticles;
+  Preslice<TrackTableMC> perCollision = aod::track::collisionId;
   void processMC(CollisionTableMCTrue::iterator const& mcCollision, CollisionTableMC const& collisions, TrackTableMC const& tracks, ParticleTableMC const& particles);
   PROCESS_SWITCH(chargedSpectra, processMC, "process mc", true);
 
@@ -178,8 +179,6 @@ void chargedSpectra::processData(CollisionTableData::iterator const& collision,
  * Entrypoint to processes mc.
  */
 //**************************************************************************************************
-Preslice<chargedSpectra::TrackTableMC> perCollision = aod::track::collisionId;
-
 void chargedSpectra::processMC(CollisionTableMCTrue::iterator const& mcCollision, CollisionTableMC const& collisions, TrackTableMC const& tracks, ParticleTableMC const& particles)
 {
   histos.fill(HIST("collision_ambiguity"), collisions.size());