adding helper functions for soft drop to be included in derivations

Similarly to other jet grooming algorithms a helper function was added
to Jetcommon.py to include jets groomed with the soft drop algorithm to derivations
using addSoftDropJets (addTrimmedJets, addPrunedJets and addFilteredJets exist already).
This addition to JetCommon.py also required changes in the JetRec tool


Former-commit-id: c8889d9be04050492c112a4aa8ff87a384574125