Product graphs

[cbbowen]

Views of two or more graphs as a single Cartesian or tensor product graph should be added.

[Fohlen]

I would strongly vow for this feature. However I would not do this begin with, it would be much better if the library would offer subgraphs (e.g minimum spanning tree) which are itself ranges over the product of the function. This would very much embrace the power of ranges v3 over the principles of BGL (which you ported so nicely).

[cbbowen]

f5028e0 added graph::tensor_product_view(...) which lays the foundation for, but it's not yet feature complete.