Add multiple tracks labels support

Summary

This MR introduces the following changes

• Add support for different labels for each track-like group
• Track labels are configurable through the GNN_Variables.yaml file in the following way:

# String with the name of the track truth variable
track_truth_variables:
  tracks : ["truthOriginLabel", "truthVertexIndex"]
  electrons : ["truthOriginLabel", "truthVertexIndex"]

This relates to the following issues

• Closes #219 (closed)
• Touches #223

I am unsure if I should change something in the documentation.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides