[yt-dev] Issue #750: Test failure for Profile1D (yt_analysis/yt)

[Kacper Kowalik]

New issue 750: Test failure for Profile1D
https://bitbucket.org/yt_analysis/yt/issue/750/test-failure-for-profile1d

Kacper Kowalik:

Reduced test case:

```
#!python

from yt.testing import fake_random_pf
from yt.data_objects.profiles import Profile1D

pf = fake_random_pf(64, nprocs=8, fields=("Density", "Temperature"))
nv = pf.domain_dimensions.prod()
dd = pf.h.all_data()
(rmi, rma), (tmi, tma) = dd.quantities["Extrema"](["Density", "Temperature"])
p1d = Profile1D(dd, "Density", 8, rmi * 0.9, rma * 1.1, False, weight_field=None)
p1d.add_fields(["Ones", "Temperature"])
```

yields:

```
#!bash
Traceback (most recent call last):
  File "issue.py", line 9, in <module>
    p1d.add_fields(["Ones", "Temperature"])
  File "yt/data_objects/profiles.py", line 760, in add_fields
    for g in parallel_objects(self.data_source._grids):
  File "yt/utilities/parallel_tools/parallel_analysis_interface.py", line 433, in parallel_objects
    oiter = itertools.islice(enumerate(objects), my_new_id, None, njobs)
TypeError: 'NoneType' object is not iterable

```