A data.frame from forecast_data that's the same as data_frame_from_forecast_data() does except
only includes point and quantile rows, and with this header: 'location', 'target', 'type', 'quantile',
'value', i.e., 'unit' -> 'location' and 'class' -> 'type'
Arguments
forecast_data
Forecast data as a list in the Zoltar standard format