moved diff_burnable_mat to model

[shimwell]

Description

It would be useful for cell based R2S simulations to be able to call diff_burnable_mats on the Model class. Then users can call model.diff_burnable_mats before calling a volume calculation. It is useful to carry out a volume calculation to get the cell/material volumes needed for depletion simulations after the materials have been diffed. Full motivation in #2714

Would it be acceptable to move the function to the model.py? We do have a few other methods that operate on materials in the Model class like update_material_volumes.

This would not change or break behavior for users that want to diff_burnablemats when calling the deplete method

Fixes # (issue)
#2714

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)

[paulromano]

Good idea @shimwell 👍 two comments below:

[shimwell]

Thanks for fixing the tests @pshriwise

Looks like this PR is now passing the tests again.

https://github.com/openmc-dev/openmc/actions/runs/6506983030