Add openmc_mesh_get_volumes C API function

[paulromano]

Description

This PR adds an openmc_mesh_get_volumes C API function and corresponding Python binding as the openmc.lib.Mesh.volumes property. I'm planning on using this downstream in the plotter application to be able to plot volume-normalized mesh tallies.

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]

Looks good @paulromano! Will merge at the end of the day if there are no other comments.