allowing different volume methods for diff mat

[shimwell]

Description

A while back I tried PR #2653 but this changed the behaviour of the diff_burnable_mat too much for it's main use case.

diff_burnable_mat appears to be doing two things when set to True, it changes the materials volumes and makes extra materials. This PR attempts to separate out both of these things and allow the user to specify each one

This PR is an attempt to add another argument to CoupledOperator so that the manner in which the volume is found can be user specified. This maintains current behavior as default when diff_burnable_mat is set to True but also allows users to split up the materials so that their volumes are assigned to the cell volume instead of always being divided equally.

This PR attempts to add another argument called diff_volume_method which can be set to "divide equally" (default) or "match cell" (useful for fusion r2s and burn up)

Fig 6 in this fantastic paper 😄 suggests we get about 20% burn up at the front of DEMO reactor solid breeder blankets. Hence I'm keen to see the CoupledOperator working in this slightly different way for fusion simulations.

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)

[paulromano]

If at first you don't succeed, try again 😄 I like the approach you have here. I just made a few small stylistic fixes and I think it's good to go. Thanks!

[shimwell]

Thanks Paul, style is always appreciated. Glad we found a method of getting the tweak to the diff_burnable_mat behaviour through, I shall be making use of this straight away