allowing mesh from_domain to use bounding boxes directly

[shimwell]

Description

I'm keen to be able to make regular meshes from bounding boxes in addition to domains that have bounding boxes. This is because I have bounding boxes for single components in DAGMC geometries and want a mesh just for that component (.e.g TF coil magnet).

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)