Implement an AgeInterval operator.

[krivit]

E.g.,

nw ~ Diss(~edges+AgeInterval(~edges, from=5, to=10))

would be equivalent to

nw ~ Diss(~edges+edges.ageinterval(from=5, to=10))

but would allow arbitrary models to be evaluated on a network constructed by taking only those edges currently present that are also of the correct time since formation.