API method for removing nodes from graph

TODO: still need to figure out what to do with relationships when doing this
Simplify group nodes transfer code

Now that we've added a bunch of wrappers around low-level operations
invovled in doing this, we can use these to abstract and simplify the
code. The old code simply served to show us where the problems were...
remove_from_group() for Groups
remove_from_group() for Data nodes
transfer_nodegraph_to_group() -> deg_group_add_nodedata()

Renamed helper function for transferring ID/Group node data over to
group to have a more consistent API
Node freeing method
Assorted fixes
* Properly clean up after freeing node data
* Removed obsolete comment