add two simple additional validation functions

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
No validation is currently performed on the contents of the annotation or the reference images.

What does this PR do?
This PR adds

• a validation function that ensures mismatches between annotation and reference image size are flagged
• a validation function that flags if reference and additional reference images contains the same data (see [BUG] both Zebrafish atlas additional references are the same as reference #230 for a real-life example where this happened)
• tests for both these validation functions, and fixtures mocking the failing situations.

References

Closes #207
Should flag #230

How has this PR been tested?

Tests added for new functionality, tests pass locally and on CI.

Is this a breaking change?

No.

Does this PR require an update to the documentation?

API docstrings added for both functions and tests.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[willGraham01]

One thing re: comparing all the elements of an atlas, otherwise some VSCode things that crept in.