Improve flexibility of IO

[clbarnes]

• Adds zarr and pyn5 (for cases where conda isn't available)
• Makes group/dataset checking more flexible by expanding register_filetype
• Reduces duplicated code in io_tests using inheritance of test classes
• Explicitly adds optional dependencies to travis environment, as they weren't being picked up before

[constantinpape]

Looks pretty cool, I will have a closer look at it later.

[clbarnes]

A bit tricky to abstract over zarr's stores given they don't have __enter__ and __exit__ methods for context manager, I've used a zarr_open function which wraps zarr.open and patches them in but I'm not happy about it.

[constantinpape]

Looks very good overall. If we can agree on a better way to structure the dependencies, we could already integrate this in this PR.

[constantinpape]

@clbarnes LGTM.
Is there anything else you want to add to this PR?

[clbarnes]

I think that's all for now! Thanks.

[constantinpape]

Thanks for the contribution!