BUG (minor) - single value data breakes generic.flag

Really minor bug in generic.flag. It breaks when single value data is passed, cause func does then not return an array:

minimal example: Data:

import pandas as pd
import saqc
s=pd.DataFrame({'a':[2]}, index=pd.DatetimeIndex(['2000']))
so = saqc.SaQC(s)

breaking call:

>>> so.generic.flag('a', func=lambda a: a>1).getResult()
TypeError: generic expression does not return an array