Unpack MCParticles when requesting MCHeader

MR following the discussion. Explicitly addresses the following issue:

We recently encountered that unpacking MCHeader does not unpack MCParticles, we had to append get_mc_particles explicitly to the algorithm "sequence" to be able to access MCParticle from MCVertex. This is a separate issue that needs to be addressed (I forgot to create an issue perhaps @sicapell already created an issue for this?)