Expose time series validation results to GUI

[paxcema]

In time series tasks, the model analysis phase should expose:

• forecasts
• actual series values
• accuracy metrics (within bounds, balanced acc score, MSE, sMAPE, etc.)

And particularly for numerical targets:

• confidence bounds (if enabled)
• anomaly tags (if enabled)

The previous info. should be computed:

• using the validation dataset
• for all groups (as determined by the group_by argument)
• for all timesteps that the user has specified (nr_predictions argument)