Add weights vector to EventHeader and constant for storing them

[tmadlener]

BEGINRELEASENOTES

• Add a double weight VectorMember to the EventHeader to allow storing multiple weights.
  • This should not break any existing code as we simply add a new member. However, this will break the possibility of reading existing files through podio.
• Introduce the edm4hep::EventWeights constant to have a consistent name for storing / accessing them in file level metadata.

ENDRELEASENOTES

• Fixes Ways to store multiple event weights #242
• Make weight a double as well? -> Yes

[tmadlener]

As discussed during the meeting on Jan 23 we also make the nominal weight of the EventHeader a double for consistency.