TriggerMenuMT: removing duplicated information on the L1decisions in the class Chain

The menu class called Chain has two internal parameters, one for L1 thresholds, and one for the L1decision collection. Only the latter is really needed. This is to cleanup the code in view of the full CA migration. Also:

• Fixed the L1Decision seeding during the merging of chains.
• Fixed the filling of sigDics to allow signatures to share the same folder (for example Egamma with [‘Photon’, ‘Electron’]). This was preventing to run on a combined photon+electron single chain.