Change XSGM interface stack position to just before before MC2

[HunterPSmith]

Description

The cross section group manager is currently called before depletion and fuel management in the interface stack. Therefore, any changes due to burnup are not accounted for when cross sections are generated.

This was historically done to support branch searching. It is also necessary to enable flux-weighted representative blocks, as several blocks will have 0 flux after fuel shuffling, so the implementation of this PR will mean that we can no longer support this.

This PR moves the cross section group manager interface in the interface stack to just before mc2. Also sets the default cross section block averaging method to Average instead of FluxWeightedAverage.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works. NOTE: PR 1308 has an additional test to support this PR.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[albeanth]

@keckler do you recall which issue you opened that discussed this? We should link that here for posterity. I tried looking for it but couldn't find it.

[keckler]

#839

[albeanth]

One minor comment here. Thanks @HunterPSmith

[albeanth]

One minor comment here. Thanks @HunterPSmith

[albeanth]

Coverage drop is addressed in #1308.

[john-science]

@albeanth Wants us to connect this PR to #1308

Essentially, that PR exists to increase code coverage that this PR drops. (Since Tony does not have the power to change this PR as it originates in a private repo.)

[albeanth]

Since Tony does not have the power to change this PR as it originates in a private repo.

Turns out I do, but I didn't find that out until after I created #1308. So instead of closing #1308 and cherry-picking over the commits or merging the feature branch to this PR, I just left everything as is....

But good to know I have some power for the future! 😄