Demo notebook accessible on colab

[ivanzvonkov]

While testing a new release of CropHarvest I found that downloading the dataset now works on Google Colab.

This PR makes the demo notebook accessible through Colab, clears notebook outputs to allow opening a clean notebook, and adds assert statements to the notebook to make it easier to test new releases.

The Open in Colab button in the README.md references the notebook in the main branch which won't be updated until this PR is merged. Here's a button to run the notebook in this branch

[review-notebook-app]

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[gabrieltseng]

LGTM!

A thought I had is that we may want a default data dir within the package, so that you could call:

evaluation_datasets = CropHarvest.create_benchmark_datasets()

and if you don't care about where the data is stored, it stores it within the package (similar to how torchvision does it).

But that would be a seperate PR / discussion.