DataArray constructor allow iterables

[clarkfitzg]

The DataArray currently requires a sequence for the coords and dims. This could be generalized to allow passing iterables. Currently this raises a ValueError:

b = xray.DataArray(np.random.randn(3, 4), dims=(x for x in ('a', 'b')))

If iterables were allowed it would work. This comes up more often in Python 3. Some conversation in #466

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