Copy warning when using the HB-method

[mfmssb]

det.hb(y_var="D_HELD", time_var="AARGANG")

yields

SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  valid_rows["ratio"] = valid_rows[time1] / valid_rows[time0]