Update DiSCOValidator to support non-ore:aggregates graph roots

[karenhanson]

The rmap-loader validation method checks that additional statements form a connected graph with the aggregated resources. A change to RMap now supports additional statements where e.g. the object is the aggregated resource - previously the entire graph had to be rooted directionally to the aggregated resources.

See this issue for more information: rmap-project/rmap#85