[Feature] Implement multi horizon model

[JanMaartenvanDoorn]

DOR:

• Custom model mechanism #230
• Implementation of grouped regressor #244

WHAT: Implement a model that trains a seperate model for each horizon.

HOW:

• Use the grouped regressor that is implemented in #244
• Use the "horizon" feature to determine which models have to be trained

TEST:

• Multi horizon model is available in for training and forecasting