Add Model Handling and Prophet implementation

[dominiquekleeven]

• Base configuration schema for models
• Prophet configuration schema
• Model training logic
• Model forecasting logic
• Store and retrieve trained models
• Store and retrieve model configurations
• Write predicted data points
• Scheduling of Training and Forecasting

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Note, the implementation should take into account the possibility that additional models will be supported in the future. It makes sense here to have a factory or such that creates a Model Provider based on the provided configuration. The provider is then responsible for dealing with the data formatting/processing, training and predict logic of the model.