Order of min/max columns in cobrapy has changed

When performing FVA in cobrapy, a dataframe is returned with the minimum and maximum flux values, each taking up a column. Currently, ensemble FVA has the column order hardcoded, but the order appears to have changed in cobrapy, or to change dynamically (unclear).

EnsembleFVA should be updated to properly assign min/max regardless of the convention in cobrapy.