Compute `last_value_datetime` using multiple output sensors

[victorgarcia98]

Where

flexmeasures.cli.data_add:add_report

What

Currently, the default start time of reports (last_value_datetime) is being computed using the first sensor in output (taking the max event_start). Nevertheless, with the introduction of the multiple-output capabilities, this query should take into account all the ouput sensors and return the minimum of the latest event_start.

This improvement would move the query into its own function and implement some tests.

last_value_datetime = (
    db.session.query(func.max(TimedBelief.event_start))
    .filter(TimedBelief.sensor_id == output[0]["sensor"].id)
    .one_or_none()
)