[yt-dev] Issue #874: YTArray multiplication is not associative with respect to code units (yt_analysis/yt)

[John ZuHone]

New issue 874: YTArray multiplication is not associative with respect to code units
https://bitbucket.org/yt_analysis/yt/issue/874/ytarray-multiplication-is-not-associative

John ZuHone:

Test case, on a FLASH dataset:

```
#!python

>>> a = yt.YTArray([1,2,3], "cm")
>>> b = ds.arr([1,2,3], "code_length")

>>> a*b
YTArray([ 1.,  4.,  9.]) cm*code_length

>>> b*a
YTArray([ 1.,  4.,  9.]) cm*code_length

>>> (b*a).in_units("code_length**2")
YTArray([ 1.,  4.,  9.]) code_length**2

>>> (a*b).in_units("code_length**2")
UnitParseError: Could not find unit symbol 'code_length' in the provided symbols.
```

Presumably this is because the final ``YTArray`` takes the unit registry from the first of the two in the binary operation. This is a simple and irrelevant example, but it illustrates the problem. Not sure how important this is. 

Responsible: ngoldbaum