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

[yarikoptic]

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 (establish testing (at least smoke) for the notebooks on the hub instance #10) and could lead to surprizes among users since behavior could depend on the sequence in which they opened notebooks