Fix @dont_isolate_yet

[jonathanslenders]

@don’t_isolate_yet

It should only be applied to a method of a ParallelNode. The API should make sure that the `self`` object looks like an unisolated node array.

Right now this is not the case when such a method is called from another method of the same node. (which is isolated) In this case, we should covert self to an array of just one Host, in order to be API compatible.