Update the return type of `get_trend_dataframe()`

[remyogasawara]

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.