Edge cases with executing all notebooks

[wrynearson]

just execute executes all .ipynb files in the notebooks directory.

This can cause issues if there are errors in execution of notebooks not relevant to the user's notebook(s) they're working on (e.g., in a PR).

Solutions

One of:

1. just execute only executes a single or list of notebooks passed as arguments (e.g., just execute notebooks/my-notebook.ipynb
2. just execute executes only unexecuted notebooks.

Option one is probably better and less complex, putting the onus on the authoring user.

Flagged in #4 (comment)