Implement predict in ensembles

[Mr-Geekman]

🚀 Feature Request

We should add predict method into ensembles.

Proposal

1. Implement EnsembleMixin._predict_pipeline.
2. Implement VotingEnsemble._predict: look at VotingEnsemble._forecast for inspiration.
3. Implement StackingEnsemble._predict
   1. Extract all logic with construction of answer in forecast into another method.
   2. Implement predict similar to forecast with preparing predictions and calling method for construct the answer.
4. Implement DirectEnsemble._predict: it should make prediction with a model with the smallest horizon.

Test cases

1. VotingEnsemble:
   1. Add test_predict_interface, look at test_forecast_interface for reference.
   2. Add test_predict_prediction_interval_interface, look at test_forecast_prediction_interval_interface for reference.
   3. Change tests test_forecast_values_default_weights and test_forecast_values_custom_weights to test _vote instead of `forecast.
   4. Add test on forecast calling _vote.
   5. Add test on predict calling _vote.
2. StackingEnsemble
   1. Add test_predict_interface, look at test_forecast_interface for reference.
   2. Add test_predict_prediction_interval_interface, look at test_forecast_prediction_interval_interface for reference.
   3. Add test_predict_values, look at test_forecast for reference.
   4. Transform test_forecast_raise_error_if_not_fitted into test that tests both forecast and predict using pytest.mark.parametrize.
3. DirectEnsemble: test that predict uses model with the smallest horizon.

Additional context

Is blocked by #958.

[Mr-Geekman]

Closed by #972.