document origin of `gdml` files

**Is your feature request related to a problem? Please describe.**
Now that we are importing `gdml` files (e.g. https://github.com/eic/epic/pull/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