hold models in ensemble configuration for multilevel optimization

Issue Description

What did you expect to see happen?

When a multiple layer optimization is performed, I would like to have the possibility to hold the outcome of a model (in the ensemble model configuration). This can be beneficial for intrinsically stochastic models.

What did you see instead?

not implemented

Do you have a suggested fix for the development team?

Please attach the input file(s) that generate this error. The simpler the input, the faster we can find the issue.

For Change Control Board: Issue Review

This review should occur before any development is performed as a response to this issue.

[x] 1. Is it tagged with a type: defect or improvement?
[x] 2. Is it tagged with a priority: critical, normal or minor?
[x] 3. If it will impact requirements or requirements tests, is it tagged with requirements?
[x] 4. If it is a defect, can it cause wrong results for users? If so an email needs to be sent to the users.
[x] 5. Is a rationale provided? (Such as explaining why the improvement is needed or why current code is wrong.)

For Change Control Board: Issue Closure

This review should occur when the issue is imminently going to be closed.

[ ] 1. If the issue is a defect, is the defect fixed?
[ ] 2. If the issue is a defect, is the defect tested for in the regression test system? (If not explain why not.)
[ ] 3. If the issue can impact users, has an email to the users group been written (the email should specify if the defect impacts stable or master)?
[ ] 4. If the issue is a defect, does it impact the latest stable branch? If yes, is there any issue tagged with stable (create if needed)?
[ ] 5. If the issue is being closed without a merge request, has an explanation of why it is being closed been provided?

idaholab / raven

hold models in ensemble configuration for multilevel optimization #271