[yt-dev] Issue #1340: create_profile returns zero-value profile when input field is negative (yt_analysis/yt)

[Suoqing Ji]

New issue 1340: create_profile returns zero-value profile when input field is negative
https://bitbucket.org/yt_analysis/yt/issues/1340/create_profile-returns-zero-value-profile

Suoqing Ji:

The test script generates a particle dataset with negative velocities. Velocity profile created from this dataset is zero. This also causes problems in ``ParticlePlot``.

```
#!python

import yt
import numpy as np

n_particles = int(1e4)

ppx, ppy, ppz = np.random.normal(size=[3, n_particles])
pvx, pvy, pvz = - np.ones((3, n_particles))

data = {'particle_position_x': ppx,
        'particle_position_y': ppy,
        'particle_position_z': ppz,
        'particle_velocity_x': pvx,
        'particle_velocity_y': pvy,
        'particle_velocity_z': pvz}

bbox = 1.1*np.array([[min(ppx), max(ppx)], [min(ppy), max(ppy)], [min(ppz), max(ppz)]])
ds = yt.load_particles(data, bbox=bbox)
ad = ds.all_data()
print (ad['particle_velocity_x'].max(),
       ad['particle_velocity_x'].min())

profile = yt.create_profile(ad,
                            ["particle_position_x", "particle_position_y"],
                            "particle_velocity_x",
                            weight_field=None)
print (profile["particle_velocity_x"].max(),
       profile["particle_velocity_x"].min())
```