Change pion implememtation to assembly from envelope volume

**Is your feature request related to a problem? Please describe.**
- Frequent overlaps between neighboring pion lucite detectors that are only caused by the envelope volumes.
- Redundant parameters like `SCALE` that don't make much physical sense

**Describe the solution you'd like**
We can use `<assembly>` tags.

**Describe alternatives you've considered**
No change.
