Backend support for Layered Networks #240

weve done good work recently introducing a Network class 🎉 but we would like to also support layered and temporal networks. my understanding is these two ideas are related in that a temporal network could be seen/ implemented as a type of layered network. but essentially were talking about a new class LayeredNetwork (or possibly two if we want a separate class and methods for TemporalNetwork, depending on use cases) w a slot called layers, which is a list of Network objects.