Graph loading not assigning proper structural types

[cdbethune]

The graph loaders have not been properly updated to work in the no-metadata context. load_edge_list, load_single_graph and load_graphs all expect there to be semantic types set on the graph metadata, and attempt to set the dataframe col type based on that. Given that there's no longer any semantic type info assigned in the datasetDoc.json, all data is treated as having type of object in the dataframe. This seems to be fine most of the graph primitives, but does seem to be breaking the vertex_nomination primitive, as the RF step is no longer completing.