Import Surface Ensemble: Depth property is set to `Fixed Depth` when importing irap files

[kriben]

Steps to reproduce:

1. Click "Import Ensemble Surface".
2. Import some irap files as ensemble.

Expected results:
When there is only on property in the file it should be selected as the "Depth Property". In irap files this would be: File Property Values.

Actual results:
"Fixed depth" is selected

[kriben]

Possible workaround from python: https://api.resinsight.org/en/main/api/rips.RegularSurface.html#rips.RegularSurface.set_property_as_depth