New metric models/tables

[peterdudfield]

We need the following new models, where we can save metrics in

metric

• id
• name
• description

datetime_interval

• id
• start_datetime_utc
• end_datetime_utc
• elexen_settlement_period (optional)

metric_value

• id
• metric_id (foreign key)
• value
• number_data_points
• location_id (foreign_key)
• datetime_interval_id (foreign_key)

[peterdudfield]

In the future we can update the forecast_value table to use datetime_interval_id instead

[peterdudfield]

This has been deployed on development