implement NGFF source validation

[martinschorb]

addresses #135

[martinschorb]

I could not find tests for the validations (checking if errors are thrown). That's a bit redundant anyways.

[constantinpape]

I could not find tests for the validations (checking if errors are thrown). That's a bit redundant anyways.

Yes, I never really got around to implement these. There's only a superficial test here.

In any case, the changes here look good and straightforward, and the code is being run by a couple of tests that add and then validate ome.zarr data ;).