Add tertiary integral to FV method

[rtimms]

Description

Adds tertiary integral to the Finite Volume method

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python -m pytest (or $ nox -s tests)
• The documentation builds: $ python -m pytest --doctest-plus src (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[rtimms]

@aabills

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.25%. Comparing base (a7253b8) to head (0042138).
Report is 18 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4710      +/-   ##
===========================================
+ Coverage    99.22%   99.25%   +0.02%     
===========================================
  Files          303      303              
  Lines        23070    23230     +160     
===========================================
+ Hits         22891    23056     +165     
+ Misses         179      174       -5     

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[aabills]

Thanks @rtimms. I can add a quick test to fix coverage if you want

[aabills]

Thanks @rtimms. I can add a quick test to fix coverage if you want

Turns out it's not possible to reach the uncovered line, so deleted it.

[aabills]

@rtimms can we merge #4665 before this, there will be a conflict that will be easier to resolve here.

[rtimms]

@aabills sounds good