Export model.tallies to XML in CoupledOperator

[gridley]

Description

The cause of the related issue was that the relatively new Model object's tallies variable was not being exported to XML. The older CoupledOperator code anticipated that users would have run tallies.export_to_xml(), but that's not how we do stuff with the Model class. Since the depletion interface now uses Model, that's a confusing expectation! It's by far best to have the CoupledOperator also export the tallies and plots file if they're present on Model.

Fixes #2839

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)

[gridley]

Since this is such a critical functionality, I've added a test to ensure we don't lose this again.

[gridley]

Not my fault on the CI failure. Any suggestions on what to do about this?

Lines executed:42.62% of 61
Submitting coverage to coveralls.io...
Could not submit coverage: 504 Server Error: Gateway Time-out for url: https://coveralls.io/api/v1/jobs
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/coveralls/api.py", line 290, in submit_report
    response.raise_for_status()
  File "/opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 504 Server Error: Gateway Time-out for url: https://coveralls.io/api/v1/jobs

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/coveralls/cli.py", line 95, in main
    result = coverallz.wear()
  File "/opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/coveralls/api.py", line 257, in wear
    return self.submit_report(json_string)
  File "/opt/hostedtoolcache/Python/3.9.18/x64/lib/python3.9/site-packages/coveralls/api.py", line 293, in submit_report
    raise CoverallsException(
coveralls.exception.CoverallsException: Could not submit coverage: 504 Server Error: Gateway Time-out for url: https://coveralls.io/api/v1/jobs
Error: Process completed with exit code 1.

[shimwell]

Slightly related but do we have plans to move away from separate tallies.xml materials.xml etc and go for the model.xml for the CoupledOperator

[paulromano]

Thanks @gridley!