self.columns doesn't always work

[cdemulde]

In the case where a column is added to the self.data dataframe, but not by a wwdata Class function, the self.columns command fails. Example:

dataset.columns

Gives ['airflow1','airflow2']
Executing:

dataset.data['airflow_total'] = dataset.data['airflow1']+dataset_control.data['airflow2']
dataset.columns

Gives the same result, while

dataset.data.columns

Does give
['airflow1','airflow2','airflow_total']