Fits-Ascii notebooks might be able to close more gracefully

One of the participants hit "too many open files" limit. I instructed the person to exit notebook session and restart it, which wasn't ideal, but fixed the problem. A better way is for all the notebooks to handle file I/O with `with` statement, which hopefully will close the file pointers gracefully (except maybe during memmap situations).

Inspect the workshop notebooks for file I/O and identify the paths that can leave file pointers open. Start by reviewing the notebook cells related to file access, then check how memmap cases differ. Done means the notebooks close ordinary file handles gracefully and the too-many-open-files problem is addressed without breaking memmap usage.

Written by the indexing model from the issue text.