[yt-dev] Issue #803: Particle filters cannot be added without referencing pf.h first. (yt_analysis/yt)

[Britton Smith]

New issue 803: Particle filters cannot be added without referencing pf.h first.
https://bitbucket.org/yt_analysis/yt/issue/803/particle-filters-cannot-be-added-without

Britton Smith:

In the experimental bookmark, adding particle filters fails unless you specifically reference pf.h first.  For example, the following script: http://paste.yt-project.org/show/4379/
results in this

```
#!python

Traceback (most recent call last):
  File "test.py", line 12, in <module>
    pf.add_particle_filter("co_star")
  File "/Users/britton/Documents/work/yt/yt-x86_64/src/yt-hg/yt/data_objects/static_output.py", line 373, in add_particle_filter
    used = self._setup_filtered_type(f)
  File "/Users/britton/Documents/work/yt/yt-x86_64/src/yt-hg/yt/data_objects/static_output.py", line 386, in _setup_filtered_type
    if not filter.available(self.derived_field_list):
AttributeError: 'EnzoDataset' object has no attribute 'derived_field_list'
```

However, this script works fine: http://paste.yt-project.org/show/4380/
The only difference is the addition of "pf.h" to line 12.