Ellipsis handling

[alimanfoo]

This PR has tests and minimal fix for bug in handling of indexing with ellipsis (#93). Also deals with case where too many index items are provided than there are dimensions. Resolves #93. Also resolves #89.

[alimanfoo]

cc @jakirkham. I know you have an open PR (#116) that resolves #93 via kenjutsu, but I'd like to keep the indexing code within zarr for the time being, so have made this PR with a minimal fix for #93. That said, if you have any comments on the implementation here, and/or advice about any other tricky indexing cases we should be covering, that would be very welcome. (I'm going to investigate fancy indexing in a separate branch.)

[jakirkham]

Not a problem. I think that PR got a bit stale. Plus there were some open questions about performance. So a simpler near term fix is reasonable and desirable.

Happy to take a look. Won't be able to do so right away, but will try to set aside some time this weekend or some time next week.

[alimanfoo]

Closing as these changes have been rolled into #172.