Make all internal columns, in particular `forecast_id`, more robust by prefixing it with "scoringutils_"

[nikosbosse]

quantile_to_interval_numeric(), for example, makes use of an internal forecast_id column. This might cause issues if there is already a forecast_id column in the input.

We should likely prefix all internal columns with "scoringutils_" to avoid those issues + document this design choice somewhere.