Parse url raise exception

[will-moore]

Fixes #471.

TODO: fix lots of tests!

We should also consider handling the scenario described there, where user expects one OME-Zarr format but gets a different one.

See #468 where we remove handling of mismatching ome-zarr formats, because we don't want to re-create a store.

probably parse_url() with No format specified should be happy to read any format, but IF you specify a format, it MUST match.
This would be similar to the strategy used for write_image() etc where the default format is None (format will be chosen based on zarr group) but if you specify a format that is incompatible, you get an exception.

e.g.

    def test_wrong_format(self):
        """ Write a v0.4 zarr, read it as CurrentFormat"""
        grp = create_zarr(str(self.path), fmt=FormatV04())
        # should use correct format
        v2reader = Reader(parse_url(str(self.path)))
        # should raise exception
        reader = Reader(parse_url(str(self.path), fmt=CurrentFormat()))
        assert len(list(reader())) == 0

[will-moore]

Closing in favour of #476