codex_ctcl.ipynb

hi Alex,

I was not sure about which tutorial to tackle first to get very in a basic way familiar with mosna.
Bram recommended me to walk through the `CODEX_CTCL.ipynb` codex tutorial.
Here are some things I caught on the flight.

Imports and data loading
1. an introduction section that people have to download the data from the supplementary data from this publication would be good: https://doi.org/10.1038/s41467-021-26974-6
1. i would avoid filename changes, or at least please mention that and how you have extended them.
1. the `raw/` sub folder seems to me unnecessary.
1. i cannot find the `cohort_response.ods` file ?

Spatial networks reconstruction
1. `select_sample` not defined.

at that point I gave up.