user stories: GDML importer for 3D geometry library

Created by: decibelcooper

The 3D library mentioned in the title is described as a basic concept in #137.

For my work, GDML will be used as a geometry exchange format. If Go is to be used for HEP/MEP event reconstruction, it will be very useful to be able to accept GDML by constructing an in-memory representation of a detector geometry with an importer. This could be a part of the library described in #137, but it doesn't need to be. For the purpose of minimizing code imports for projects that don't need GDML import, I think it makes sense to have this separate.