Add test fixtures for cell matching regression test

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
As discussed in brainglobe#74, we'd like a way to access the example cell data from the tests, without clogging the github repo.

What does this PR do?

• Adds a pooch registry, to make a start moving all BrainGlobe test data to GIN
• Adds a fixture to access the cell data to write a regression test for cell matching.

References

Makes a start on brainglobe/BrainGlobe#5

How has this PR been tested?

I've checked locally that printing the cells after getting them via the fixture works as expected.

Is this a breaking change?

No

Does this PR require an update to the documentation?

Already documented in our dev guide (but could do with an example, or a link to our HowTo for GIN, I guess?)

Checklist:

• The code has been tested locally (manually)
• [n/a] Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit