document origin of gdml files

[c-dilks]

Is your feature request related to a problem? Please describe.
Now that we are importing gdml files (e.g. #379 and #382), we should try to keep track of where they came from.

Describe the solution you'd like
Documentation or links to other repositories would be good; we may need to modify these gdml files in the future, and the easiest way to do so is by modifying the code that was used to produce them.

Describe alternatives you've considered
No documentation and no clear origin of gdml files