Make notebooks runnable on Google Colab

[redeboer]

The notebooks currently don't automatically run on Google Colab, because dependencies are not installed and ipympl needs to be manually activated. One could do this interactively in a cell before the cell with imports as follows:

import os
if "COLAB_BACKEND_VERSION" in os.environ:
    import subprocess
    from google.colab import output
    dependencies = [
        "ampform",
        "graphviz",
        "ipywidgets",
        "ipympl",
        "tensorwaves[jax,pwa]",
    ]
    subprocess.check_call(["pip", "install", *dependencies])
    output.enable_custom_widget_manager()

It works, but it is not pretty, because it's is easy to forget to update it if the requirements (imports) for the notebooks change. But just an posting here for documentation.