Remove unused materials definitions from final gdml

[lobis]

Currently materials loaded from a URL via loadMaterials (src/jvmMain/kotlin/space/kscience/gdml/loadMaterials.kt) are added to the <materials> section of the final GDML.

This creates a nuisance when using the GDML in Geant4 as Geant4 will (only on some physics lists such as decay) load cross sections for all isotopes which can take a few extra minutes in my experience.

The solution would be to identify materials used in the geometry and only keep those, while also keeping child materials/isotopes/elements. It would also reduce the size of the file and make it more readable.

I will try to implement this if nobody is available.

Edit: I am currently working on this.