Use standard locations when packing MC objects

Related to Analysis!119 (merged), where @ibelyaev and I found out that the MC particles and vertices were being packed to non-standard locations, this MR puts the packed objects in the default places. Backwards compatibility for reading files made before this MR is implemented in Analysis!119 (merged). I can't find any other relevant instances of pMC in the code, so I think that and this MR should cover it.

/cc @jonrob