edge cases for cleaning .mbox

[isabelizimm]

Describe the bug
Some .mbox files are a list of strings and nested list, rather than just list of strings. This causes issues when using cleaning functions.

To Reproduce
Steps to reproduce the behavior:

1. Go to ./notebooks/01_collect_data/raw_to_text.ipynb
2. Run notebook end-to-end, using the entire variable mboxes rather than subset_mbox in IN[5]
3. See issue below

Expected behavior
Expected to run through all .mbox files.

Screenshots

Additional context
Here is screenshot looking inside a problematic .mbox