Include weight information for importance sampling in GDML

[Cédric Hernalsteens]

The idea is to merge the weight information (currently in a text file) within the GDML file describing the parallel world definition. The auxiliary data in the GDML standard could be used for that purpose, see Section 3.5.6 of the GDML user manual.

[Laurie Nevay]

Sounds good - go for it! A few things that shouldn’t change:

• loading of importance values from a file should remain (does this take precedence if both are present?)
• gdml should of course load without it with no problems
• we have some auxiliary tag info loading already for colour information although this is maybe tenuous (badly formed hand-written example just now)

I would say we also add to pyg4ometry similarly so we can write the values (if we can’t already).

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[Laurie Nevay]

Assigning to Will to decide on whether to do or not.