allow user-settable splitting of classes

i think there should be a way to have some classes (a) labeled, like the root markers, and (b) represent some set of nodes, **in such a way that those nodes will remain separated from another class** even if they would otherwise be merged something like ``` +-<DE pkgs> <gen. 123>-+-<heavy pkgs> +-<other> ``` instead of ``` <gen. 123>-+-<almost all pkgs> +-<like 3 pkgs, referred by some root somewhere> ``` reasoning: something i thought would be a good use for this tool is to see which packages are drawing the most - and the heaviest - dependencies, so i can see if there are some packages which i don't really need but are pulling heavy dependencies, or being a near-duplicate of an existing dependency, etc. it might be a problem to ensure this doesn't break the tree-like nature of the graph, though... tried asking in the nixos matrix server but got no answer aside from being sent here, so at the very least this will require more documentation.