PWGHF: update of DDbar correlation code (remove obsolete tasks + add process switch) (#515)

* PWGHF: update of DDbar correlation code (remove obsolete tasks + add process switch)

* Modifications related to review comments

* Addressing comments by Vit

* Small naming fix from Vit's comment

* Moving to McParticle_000

* Using variables for histogram binning definition

Author: fcolamar