Refactor Xenium + Visium paths and use better names

[LucaMarconato]

• [ ] all the paths used in notebooks 00_xenium_and_visium.ipynb, 01_xenium_and_visium.ipynb and 02_xenium_and_visium.ipynb should be placed in a config.py file
• [ ] sandbox.zarr was a poorly chosen name, rename it to manual_annotations.zarr. I have already made a copy in the S3 bucket of that file (so that now both sandbox.zarr.zip and manual_annotations.zarr.zip are present), so one needs just to adjust the notebooks.

[melonora]

@LucaMarconato Please see last comment in #36