Minor refactoring and start of test coverage for zoo

[tennlee]

Work still under way
Some refactoring of classes within Zoo
Some unit tests added (more work required)

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[tennlee]

Hi @edoyango I have started on some work to add a bit of test coverage to the zoo package. This includes some minor refactoring to start to simplify the import structure. I'm not quite sure whether to merge early and then keep going, or keep working away at this until the tests are more rigorous and provide more coverage. Let me know what you think of things, I could go either way.