Add nested set model implementation of hierarchical model

[rcxwhiz]

Tests are currently passing. At this point I need to:

1. Put in an advanced delete nodes test
2. DONE Look into refreshing the ._parent member of the adjacency list model, which can be set to null after a delete
3. DONE In general organize / clean up the methods
4. DONE Possibly put in some tests for the NSM specific features like num children

[rcxwhiz]

5. DONE Add a .set_parent_unchecked() to the ALM

[rcxwhiz]

All remaining items finished.