Use constants when applicable

[eKathleenCarter]

Most of the node types, attribute names, and other strings that should be constants defined in one place are set up correctly now, but not all of them. Specifically, as mentioned above, nodes are sometimes represented as dictionaries and the properties "id" "name" and "category" are hard coded in many places. Using objects would solve this but something like "category" should not be in multiple places in the code base, it should be a constant.

[eKathleenCarter]

This is mostly done. Just needs reviewing and confirming with a quick inspection of the code.