[yt-dev] Issue #839: AMRKDTree.count_cells() fails with a YTUnitOperationError (yt_analysis/yt)

[Stuart Mumford]

New issue 839: AMRKDTree.count_cells() fails with a YTUnitOperationError
https://bitbucket.org/yt_analysis/yt/issue/839/amrkdtreecount_cells-fails-with-a

Stuart Mumford:

running `amrkdtree_downsampling.py` produces the following:

```
#!python
Traceback (most recent call last):
  File "/home/stuart/BitBucket/yt/doc/source/cookbook/amrkdtree_downsampling.py", line 19, in <module>
    print kd.count_cells()
  File "/home/stuart/BitBucket/yt/yt/utilities/amr_kdtree/amr_kdtree.py", line 537, in count_cells
    return self.tree.sum_cells() 
  File "/home/stuart/BitBucket/yt/yt/utilities/amr_kdtree/amr_kdtree.py", line 124, in sum_cells
    li = np.rint((nle-gle)/dds).astype('int32')
  File "/home/stuart/BitBucket/yt/yt/units/yt_array.py", line 496, in __rsub__
    lo = sanitize_units_add(self, left_object, "subtraction")
  File "/home/stuart/BitBucket/yt/yt/units/yt_array.py", line 141, in sanitize_units_add
    dimensionless)
yt.utilities.exceptions.YTUnitOperationError: The subtraction operator for YTArrays with units (code_length) and (1) is not well defined.
```