Avoid potentially unwanted resampling of parameters

[oyvindeide]

EnsembleExperiment has several different use cases:

1. Running sensitivity analysis, in this mode, the users have external parameters to ert, and ert is mainly used as a workflow manager, sampling of prior may or may not be important
2. Evaluating prior for assisted history match when not doing e.g ES-MDA, in this context, we would like ert to sample the prior
3. Evaluating posterior after performing a manually triggered run of the update algorithm, we dont want ert to sample the prior as that would cause it to overwrite the updated parameters

There is no way for the user to toggle which use case they are operating on, and the logic in ert to differentiate is here:

ert/src/ert/run_models/ensemble_experiment.py

Lines 140 to 141 in 2bc649d

	if not prior_context.sim_fs.realizations_initialized(
	prior_context.active_realizations

We should make that more explicit, there are multiple ways of doing that, either adding a check box with Sample parameters, or splitting it up in several run models, or perhaps other. This should be coupled with a check if we will overwrite parameters, and a warning if we do so.