[yt-dev] Issue #1058: [BUGFIX] ART star particle creation time field (yt_analysis/yt)

[Kenza Arraki]

New issue 1058: [BUGFIX] ART star particle creation time field
https://bitbucket.org/yt_analysis/yt/issues/1058/bugfix-art-star-particle-creation-time

Kenza Arraki:

dd[('stars', 'particle_creation_time')] fails

```
#!python

YTUnitOperationError: The <ufunc 'subtract'> operator for YTArrays with units (dimensionless) and (code_time) is not well defined.
```


The 'particle_creation_time' field has been incorrectly given 'code_time' units.
The 'current_time' attribute needs to return a time with physical units.

Responsible: karraki