load mesh objects from weight_windows.h5 file

[GuySten]

Description

Currently, when openmc load weight windows from an hdf5 file the meshes are not loaded.
This PR change this behavior so the meshes inside the weight windows hdf5 file are also loaded.

Fixes #3588

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) 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)

[pshriwise]

Thanks for this, @GuySten. One comment on separating concerns. We'll also want to attempt some testing for this of course if you're up for taking a crack at that.