Consider adding filter realizations by sensitivity to RealizationFilter

[jorgenherje]

Consider adding support/mode to filter/find realizations based on selected sensitivity/sensitivities.

• This could be a way to define a sensitivity ensemble.
• If we implement functionality for "pairing" an ensemble and a realization filter as an entity, we can say that a sensitivity ensemble is an ensemble + a realization filter filtered on selected sensitivity.
  • Ref.: Have RegularEnsemble/DeltaEnsemble and corresponding RealizationFilter as an ensemble entity? #828