Support multiple meshes in R2S calculations

[paulromano]

Description

This PR extends R2SManager to accept a list of meshes (as the domains argument), enabling mesh-based R2S calculations to use multiple non-overlapping meshes in a single workflow. Each element-material combination across all meshes is treated as an independent activation region. This can come in handy in a variety of situations:

• Problems requiring varying spatial resolution in different regions
• Activating two or more disconnected regions where a cell-based workflow is not sufficient
• Activating a component that covers a large physical extent but is mostly empty (e.g., the vacuum vessel of a tokamak)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• 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)