Support stratigraphic mapping of intervals covering multiple stratigraphic intervals

[tnatt]

It is a very common use case to have intervals in the model that covers multiple stratigraphic units in the stratigraphic column. e.g. often found for intervals in the velocity model or the zones in a grid.

Per today it is only possible to map an interval name to a stratigraphic interval if it covers one unique interval in the stratigraphic column.
If the interval covers multiple stratigraphic intervals, the data.name is not converted to a stratigraphic name. And data.stratigraphic is set to false.

We should support this use case.

Probably that involves not converting data.name into the official stratigraphic name, related #851.
We could consider linking only to the stratigraphic top and base horizon instead of the names of the stratigraphic units. Then it can be inferred from the stratigraphic column in smda which intervals it covers.
Or we should support having multiple items under stratigraphic_entity