Throw an error when concatenations have different number of children

[aabills]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #4561

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 run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ 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

[aabills]

Tests are going to fail on this due to an unrelated issue that I am working on concurrently, so opening this for now and once we merge the other one we can merge this one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.26%. Comparing base (1022c6b) to head (f313159).
Report is 165 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4562      +/-   ##
===========================================
- Coverage    99.27%   99.26%   -0.02%     
===========================================
  Files          302      302              
  Lines        22868    22889      +21     
===========================================
+ Hits         22703    22721      +18     
- Misses         165      168       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kratman]

Seems fine as a temporary fix for the bug

[kratman]

@aabills Can you update the bug fix section of the change log

[kratman]

Looks good to me now. I can't vouch for the physics, but I don't see any issues with the code.

[aabills]

@aabills Can you update the bug fix section of the change log

Done, I realized I haven't done that for the other PR's either so I did all 3.