Pluginsv2

Version 2 of the treemachine plugin services. Allows querying the graph and synthetic tree (see TEST.md for examples).

Code does not involve/require any changes to the treemachine DB.
It's generally better to make the return type more general--in this case List<String>--so that code calling this method does not rely on the expectation of a certain type of List. There are several instances of this in preceding methods as well.