Implement Masked / Stacked Graphs

[camillescott]

Currently, many traversal and other functions have a lot of duplicated code to deal with masked (or stacked) graphs. I'd like to replace all those overloads by introducing Masked and Stacked graphs: they'll subclass the existing dBG, and take a dBG in their constructor, using the existing storage from the given dBG. They then simply reimplement the query method to do extra checks before / after the main storage query.