Precondition Node should run children to completion before checking it's if statement again

[dsobek]

I ran across an issue where a Precondition's "if" statement was evaluating from true to false while it's children were still executing. This resulted in the children only executing partway before the Precondition node short circuited the tree by returning the value of the "else" port unexpectedly.

For a more concrete example, suppose I want to implement a "RunOnce" behavior as a subtree, the xml would look like the following:

<Sequence>
    <Script code="first_run := true" />
    <Precondition if="first_run == true" else="FAILURE">
        <Sequence>
            <Script code="first_run := false" />
            <SomeLongRunningAction />
        </Control>
     </Decorator>
 </Control>

Over multiple ticks of SomeLongRunningAction this will shortcircuit return FAILURE from the "else" of Precondition. This PR would allow for SomeLongRunningAction (and any other following nodes that are children of Precondition) to run to Completion.