Update the return type of `get_trend_dataframe()`

Currently, in the `STLDecomposer`,`get_trend_dataframe()` returns `list(pd.DataFrame)` for single series and `dict(list(pd.DataFrame)` for multiseries where each key in the dictionary maps to a list that has a single dataframe that holds the signal, trend, seasonality and residual. In both cases the list only holds a single dataframe in order to stay consistent with the `get_trend_dataframe()` function in the `PolynomialDecomposer` which is not yet extended to support multiseries and returns a list with a single dataframe for single series and a list with multiple dataframes for multiseries.