[OpenSTEF 4.0] Fully support multihorizon forecasting

[egordm]

As OpenSTEF
I want to support multihorizon forecasting
So that I can create more accurate forecasts

🌍 Background

Important

Changes should be done by forking OpenSTEF 4.0.0 tracking branch (release/v4.0.0) and then make a PR back onto release/v4.0.0 branch.
Please read the contributing guide before you start.

❗Priority (What if we don't do this?/Are there any deadlines? etc.)

Definition of Done:

✅ Acceptance criteria

• A combined model is created which splits horizon across multiple models (some models forecast only horizon 0-24h for example)
• The predictions are stitched together
• Data availability is handled correctly

📄 Documentation criteria:

Interfaces with other products and services

Update relevant confluence pages

🧪 Test criteria:

Unit tests written and passed

⌛ Dependencies:

N/A

🚀 Releasing:

N/A

Other information: