Implement LocalEnsemble finalize function, which combines all realisations into one dataset.

[oyvindeide]

This has potential performace improvements. We should do it explicitly in the run models after we have finished loading all data. Could potentially be done in a different thread (or process). Then load results manually should probably do this directly and not do the callbacks individually first.

[pinkwah]

Gonna wait with implementing this until we have figured out the starting on an existing case issue.

[oyvindeide]

First step should be testing the performance of loading data from a finalized dataset compared with loading from multiple. Hopefully using a finalized dataset should take advantage of lazy loading. We should also check on NFS. Summary data is the most important to check. They are currently saved under a summary group, and the least performant at the moment, as we load all datasets and just pick out a single key. If this is not significantly more performant with a finalized dataset we need to revisit the way we store summary data.

[eivindjahren]

Resolved by #7563