Fix seasonality extension.

[tjphilpot]

This can append too many months to the seasonalities. In the case of 'Monthly' data, config['output_size'] is 18, config['seasonality'] is 12. If seasonalities_stacked.shape[1] is 72, start_seasonality_ext is 60, so line 89 appends 12 seasonalities, rather than 6 (as in the C++ dynet code).

 //if prediction horizon is larger than seasonality, so we need to repeat some of the seasonality factors
                if (OUTPUT_SIZE_I > SEASONALITY) {
                    unsigned long startSeasonalityIndx = season_exVect.size() - SEASONALITY;
                    for (int i = 0; i < (OUTPUT_SIZE_I - SEASONALITY); i++)
                        season_exVect.push_back(season_exVect[startSeasonalityIndx + i]);
                }

This becomes a problem on line 126 where we select the last config['output_size'], which will select the wrong output_size elements. In the case above, the seasonalities will be shifted by 6 months (18 - 12 = 6).