Augment auxiliaries API to enable "eager" evaluation.

[krivit]

If a term that uses an auxiliary requires information produced by initialising that auxiliary, it would be efficient if the term could initialise the auxiliary and then instead of returning an auxiliaries= formula return a list of initialised auxiliaries.

The immediate application is statnet/ergm.multi#29: to identify which network(s) should be used in submodel initialisation, we need a list of observed layers involved in a particular layer logic formula.