revisit getParents and getDeps #49

These are essentially fully set up, but some questions are arising.

1. At the moment, these are framed as returning varRanges rather than nodeRanges. However what is returned are fundamentally nodeRanges as I pass results of graphRules through the declRules in order to exclude RHS only.
2. Given that nimble returns nodes, I think we should probably do the same in nimbleModel.
3. I haven't yet implemented various args that are present in nimble, namely determOnly, stochOnly, includeData, dataOnly, includeRHSonly, predictiveOnly, includePredictive.

So I think the path forward is to pass results through getNodes, including the various "include/only" flags. This will also address duplication (issue #36) once I add deduplication in getNodes.