Updated multi-particle detection tutorial

[Pwhsky]

Updated the particle detection tutorial that uses a U-net and DeepTrack2 augmentations.
The dataset can be downloaded automatically by running the cells at the start.

[mirjagranfors]

I think that we only need to have the pip install deeptrack if running on Colab, so I would suggest to change the first line to # !pip install deeptrack # Uncomment if running on Colab/Kaggle..

I would also suggest to add a sentence about the dataset that you are loading in the beginning of the script, to make it clear that these are experimental images where you want to find the positions of the molecules/particles. And that you will use simulated images to train your NN, and then will test the NN on the experimental images.