[yt-dev] Issue #923: to_pw should raise an error if it would produce a plot with an empty field list (yt_analysis/yt)

[Nathan Goldbaum]

New issue 923: to_pw should raise an error if it would produce a plot with an empty field list
https://bitbucket.org/yt_analysis/yt/issue/923/to_pw-should-raise-an-error-if-it-would

Nathan Goldbaum:

This can happen if a user does not preload a field on a data source and then calls `to_pw` with no arguments.

Responsible: ngoldbaum