[yt-dev] Issue #937: to_pw silently drops some fields (yt_analysis/yt)

[Douglas Rudd]

New issue 937: to_pw silently drops some fields
https://bitbucket.org/yt_analysis/yt/issue/937/to_pw-silently-drops-some-fields

Douglas Rudd:

to_pw silently drops fields from frb._exclude_fields and _key_fields without notifying the user.

```
#!python

>> ds = yt.load("outputs/sedov_000960.art")
>> fields = ["density","temperature","dx"]
>> plot = ds.slice(2, ds.domain_center[2]).to_pw(fields)
>> print plot.fields
['density', 'temperature']
```

In this case ('index', 'dx') is in both exclusion lists. Any reason I can't plot a slice of my resolution?