Delta ensemble/case: Freeze for delta Everst batch case

[hhgs]

When investigating crash for delta ensemble for Everest batches, a single case delta-case froze when updating curve from realization 0 to 1 for the reference case.

Steps

1. Load Everest batch ensemble
2. Make delta summary case
3. Select realization 0 for both cases in delta calculation
4. Update both cases to realization 1
   --> freeze

[magnesj]

Not able to reproduce.