KeyError in dictionary for pipeline transformation on new data for single series

[remyogasawara]

After extending the STLDecomposer to support multiseries and changing the class variables to dictionaries, there was an error in pipeline_tests/test_pipeline_utils.py where the keys of the series do not match for single series. Currently, we access the values by casting it as a list and then indexing, but it should be further investigated to simplify the code block.

if len(y.columns) > 1:
    seasonality = self.seasonalities[id]
    trend = self.trends[id]
    residual = self.residuals[id]
    period = self.periods[id]
else:
    seasonality = list(self.seasonalities.values())[0]
    trend = list(self.trends.values())[0]
    residual = list(self.residuals.values())[0]
    period = list(self.periods.values())[0]