MTD simulation: add support for pre-mixing in pileup management of MtdSimLayerCluster

[fabiocos]

PR description:

The MtdSimLayerCluster collection, used to encode MTD PSimHit clusters and associate them to both TrackingParticle collection and MTD reconstructed hits, was tested to properly handle pile-up in the classic MixingModule case, but was not adapted for pre-mixing.

This PR adds support for the pre-mixing case, and adds a simple analyzer to directly inspect the content of the MtdSimLayerCluster collection.

This addition can possibly serve as a basis for further extension to other MTD-truth collections.

PR validation:

Code compiles, run, and has been tested with the dumper code to combine signal and pile-up in test wf 25034.999.