A more cohesive set of test fixtures

[niksirbi]

@lochhh commented on #49:

It's very nice that we now have a sample DLC project in test data. I'm guessing the broken frame files are intentional, but it would be better to have actual frames, since this would allow us to have images with non-zero width and height in the COCO JSON files. As a follow-up PR, we should consider adding the generated benchmark dataset(s) as part of our test data. These could replace the current "reference dataset" Train/SWC-plusmaze/sub-M708149_ses-20200317/, giving us a more cohesive and internally consistent set of test fixtures (using the sample DLC project as test inputs and the reference dataset as expected outputs) - our current test data comes from a mix of sample inputs/outputs across different projects, along with multiple ad‑hoc fixtures.

We should replace tests/data/Train/SWC-plusmaze/sub-M708149_ses-20200317/ with the outputs we generate in the example for M708154 and/or M727755 (sample dlc data added in this PR). This would allow us to test conversions end-to-end with actual inputs and the "converted benchmark dataset", besides being the reference/sample, functions also as the expected outputs. That said the current unit tests sufficiently cover the logic. I'm happy for this to be done in a follow-up PR.

I agree with this suggestion, we should use the same reference project both on the input side (DLC format) and on the output side (poseinterface format).

We should also re-examine if we still need all the test data files:

Since we have 2 CollectedData.csv, shall we use single-index DLC format for one and multi-index for the other? We could potentially remove CollectedData_Pranav.csv and CollecetdData_shailaja.csv from our test data