Concept requirement for load_vertices and load_edges fail in implementation

[pratzl]

load_vertices() has requires copyable_vertex<invoke_result<VProj, ranges::range_value_t>, VId, VV> and load_edges() has a similar requirement, but callers fail when enabled.

Using them as-is for the paper is sufficient for now, but we need to track that down. It may be in subtleties, like moving a string name from the original vector into the graph.

[pratzl]

The implementation has both a const VRng& and VRng&& versions. The const version may be sufficient.