Unified converter maps

[tmadlener]

BEGINRELEASENOTES

• Introduce a global object map that is shared by all converter instances (in either direction). This is necessary to properly set all relations even if the related objects are not converted in the same converter instance.
  • Fixes key4hep/k4LCIOReader#31
  • Fixes Broken associations when using edm4hep input  #113
• Add a global_converter_maps test case (and a description) that checks that this works as expected.

ENDRELEASENOTES

New iteration of #144 (see more discussion there)

This should solve #113 in the end.

• Needs Make it possible to share maps between both directions k4EDM4hep2LcioConv#21
• Includes Add some more outputs to the converter tests #143 (can be removed if that is not merged)
• Includes Remove EDM4hep -> LCIO (and back) converter test #146
• Resetting the global maps for each event.
• Release notes
• Tests (added "minimal" test case + description)