Consistent handling of 0-dimensional XArrays for dtype=object

Numpy unpacks 0-dimensional ararys when indexed. We should do the same.
Clearer test-cases for 0-dimensional data tests
Removed unused assertion method
Removed unused import
Merge pull request #44 from akleeman/fix-0d-xarrays

Consistent handling of 0-dimensional XArrays for dtype=object