Use snowglobes_data package instead of hardcoded path

[JostMigenda]

Closes #208. I’ve just filed SNOwGLoBES/snowglobes#21, which would help publish SNOwGLoBES data files as a PyPI package. This would simplify snewpy installation (users no longer need to download SNOwGLoBES manually) and usage (no need to specify the SNOwGLoBES directory in snewpy.snowglobes function calls).

To test this:

1. Run pip install -i https://test.pypi.org/simple/ snowglobes_data
2. Then install snewpy from this branch
3. Open SNOwGLoBES_usage.ipynb and set SNOwGLoBES_path to None.

This PR is currently marked as a draft, because there are a few open To-Dos:

• [x] Update documentation
• [x] Update example notebook
• [x] Remove need for backported importlib_resources library under Python 3.9
• [x] Wait until SNOwGLoBES PR is merged & package is available on regular PyPI.

[Sheshuk]

I tested and it works for me. Also I think that's a really nice improvement for the SNEWPY usability - it will allow pip installing it with all the dependencies rightaway 👍🏻

[mcolomermolla]

A couple of notes after testing:

• To be added on the doc: If the user has the $SNOWGLOBES environment variable, by default if goes to the snowglobes installation, and does not use the snowglobes_data package. If $SNOWGLOBES not set, then uses snowglobes_data.
• For using it with the $SNOWGLOBES environment variable, Jost is going to fix the current bug.

[JostMigenda]

The SNOwGLoBES PR is now merged and snowglobes_data is published on PyPI. With that, I think this PR is ready.

@mcolomermolla Can you please confirm whether I’ve resolved all your comments?