Remove all test data from repositories

[dstansby]

So we can package the test scripts, and have them accesible outside of the git repositories (e.g. for running the tests when packaging for conda-forge), a first step is to remove the test data from repositories to another place (https://zenodo.org/ is what I've used before, but there are probably other options?) and then use pooch to manage sample data download and caching.

[adamltyson]

Everything is on GIN currently, so I'd like to either stick with that, or move to something else for all BG data. GIN has worked well, but there may be a better option.

[alessandrofelder]

Related issues

• Update the sample data cellfinder#305
• Move test data to gin and fetch via pooch cellfinder#282
• Store sample data on GIN and fetch with pooch brainglobe-registration#20
• Agree on large data to be used for end-to-end tests brainglobe-workflows#19