Add support/mounting structure to detector geometry description

[bryngemark]

A lot of our detector components hang in the air. Or, is that still vacuum? 🤔
This doesn't really contribute to the realism of the radiation environment in which we're trying to identify DM.

Describe the solution you'd like
For better realism when it comes to scattering and all kinds of junk, it would be great to

• have some rudimentary implementation of tracker plate, TS plastic mounting, target mounting, ...
• and also use air around it if we don't already.

Describe alternatives you've considered
To live my life in an ideal world. Tempting