Support Everything that XArray Expects

https://github.com/bolt-project/bolt/issues/58

• single argument ufuncs (sin, exp, etc.) and ufunc like functions (pd.isnull, notnull, astype, around, isclose)
• broadcasting binary operations (e.g., ndarray arithmetic)
• three argument version of where (preferably with broadcasting)
• aggregation methods
  • max/min/sum/prod
  • argmax/argmin/std/var
  • nan-skipping aggregations (e.g., nanmin, nanmax, nanprod, nansum)
• indexing with integer arrays, booleans, slices, None
• transpose
• indexing:
  • basic indexing (int/slice)
  • outer indexing for a single array
  • outer indexing (int, slice and 1d integer arrays separately applied to each axis)
  • vectorized indexing (integer arrays with broadcasting, like NumPy)
• broadcast_to (NumPy 1.10)
• concatenate and stack (NumPy 1.10)

Start with the linked bolt-project issue #58 and review the checklist, focusing on the remaining vectorized indexing item. Confirm the expected behavior against the XArray and NumPy compatibility requirements listed here; done means the unchecked requirement is implemented and the full checklist is satisfied.