Small refactoring to detector evaluation

[sfmig]

Description

What is this PR?

• [ ] Bug fix
• [ ] Addition of a new feature
• [x] Other

Why is this PR needed? Small edits to the evaluation code from reviewing #180

What does this PR do?

• define frames_output_dir as an evaluation interface attribute
• update readme to refer to other guides

References

Part of the work from #180

How has this PR been tested?

\

Does this PR require an update to the documentation?

\

Checklist:

• [x] The code has been tested locally
• [ n\a ] Tests have been added to cover all new functionality
• [x] The documentation has been updated to reflect any changes
• [x] The code has been formatted with pre-commit

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 47.97%. Comparing base (420e0f7) to head (849940f).

Files with missing lines	Patch %	Lines
crabs/detector/evaluate_model.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #240 +/- ## ======================================= Coverage 47.97% 47.97% ======================================= Files 24 24 Lines 1532 1534 +2 ======================================= + Hits 735 736 +1 - Misses 797 798 +1 ```

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