Make notebooks "portable" (i.e. to be used not only on the hub)

Notebooks should be usable on any "compatible" system: user laptops, etc.

- Decide on how to instrument a download location for the data in the notebooks .
- Inspired by `mkdir ../data` in https://github.com/dandi/example-notebooks/pull/8/files#diff-69cfed3e5f1ce14dd4a958d296e0f62cfee6fb1b4208c2acc6daf1901b3130e4R146
- Most likely should be a `data/` subfolder within this repository as a placeholder (or potentially some data under git-annex) for the location of downloads.
- Some notebooks suggest/use `~/Downloads` as the default one where the browser would download data
- Strive to avoid modification of the environment from within the notebook
- Some notebooks have `!pip install -r ...` commands, which alters current environment
- Although "user friendly" somewhat -- complicates testing (#10) and could lead to surprizes among users since behavior could depend on the sequence in which they opened notebooks