Relabel subcomponents of SCDL node models ...

... SCDL nodes were labeled using terms derived from "input" and "output". However, this is extremely confusing: pins are labeled for external use (i.e. a sink is labeled as an input). However, if an input is referenced by a SCDL node from within the scope that defines the pin, then the input pin is actually a _source_ (driven by some upstream source). So I'm renaming nodes using "source" and "sink" nomenclature to make derived code more readable.