Adding HySGen to examples, and the hypergraph class (HGraph) to snap-core graphs.

[bpedrood]

• glib-core/bd.h: Added a struct __exception to make the hysgen/HGraph work with ubuntu >= 18.04.
• snap-core/gbase.{h,cpp}: Added flags and strings to register the hypergraph class as a standard graph class.
• snap-core/graph.{h,cpp}: Added the Hypergraph class THGraph, its pointer, and its methods' implementations.
• snap-core/graph.{h,cpp}: Added a declaration and implementation of the subgraph extraction method (GetSubGraph) for THGraph.
• Added hysgen (Hypergraph Simultaneous Generators), a probabilistic generative model for discovering node clusters/communities in hypergraphs, to examples/.
• Added snap-adv/hysgen.{h,cpp}, which includes the main library of classes and functions required to run the examples/hysgen project.
• Edited examples/Makefile to add hysgen to the examples' make routine.

[roks]

Thank you very much for your contribution!