Add single-clad material to geometry

**Is your feature request related to a problem? Please describe.**
I'm trying to add single-clad material to geometry

**Describe the solution you'd like**
I'm considering to modify 'single clad thickness'.

**Describe alternatives you've considered**
TBD

**Additional context**
This is one of the main tasks in to-do list.

The issue names no files, tests, or entry points; begin by locating the geometry definition and the existing “single clad thickness” setting. Clarify how single-clad material should be represented and what geometry behavior is expected before implementation, since the issue does not define a completion check.

Written by the indexing model from the issue text.