provide functionality to get demo data from within python

for lib5c specifically, addition of this functionality would allow the tutorials to run on windows (tox currently skips them on windows)

it is also tangentially related to #72, because in the limit we could delete all test data from the repo and download it at test time. my current feeling is that this would be overkill - it seems simpler and “lightweight enough” for us to provide very small files in test/ for testing