DataSet::guessChunking: Fix code to handle zero extents

The initial for_each for was done in the passed NDSize (dims) not
the resulting chunks variable. This is clearly a mistake. The copy
of the function parameter inside the function is no needed at all.
[test] DataSet: test for creating DataSet with zero extents
Merge pull request #295 from gicmo/dataset_zeroextent

Allow creating DataSets with zero extent