Have EquilibrationLayer not return "unsuccessful" properties

[lilyminium]

As the EquilibrationLayer in #607 is currently implemented, properties are returned as "unsuccessful" once the equilibration has run properly. I think this is necessary because otherwise the property will get removed from the "to-compute" queue and not get passed to future PreequilibratedSimulationLayers. However, it's pretty confusing to the user. Not sure if this is possible and either way it's probably not easy, but it would be neat to return properties as successful but still keep them in the to-compute queue.