Triad census of a graph.

This is a pretty simple class - basically it is a tuple, but it allows the user to refer to its individual items by the following triad names:

• 003 -- the empty graph
• 012 -- a graph with a single directed edge (A --> B, C)
• 102 -- a graph with a single mutual edge (A <-> B, C)
• 021D -- the binary out-tree (A <-- B --> C)
• 021U -- the binary in-tree (A --> B <-- C)
• 021C -- the directed line (A --> B --> C)
• 111D -- A <-> B <-- C
• 111U -- A <-> B --> C
• 030T -- A --> B <-- C, A --> C
• 030C -- A <-- B <-- C, A --> C
• 201 -- A <-> B <-> C
• 120D -- A <-- B --> C, A <-> C
• 120U -- A --> B <-- C, A <-> C
• 120C -- A --> B --> C, A <-> C
• 210C -- A --> B <-> C, A <-> C
• 300 -- the complete graph (A <-> B <-> C, A <-> C)

Attribute and item accessors are provided. Due to the syntax of Python, attribute names are not allowed to start with a number, therefore the triad names must be prepended with a lowercase t when accessing them as attributes. This is not necessary with the item accessor syntax.

Examples:

>>> from igraph import Graph
>>> g=Graph.Erdos_Renyi(100, 0.2, directed=True)
>>> tc=g.triad_census()
>>> print(tc.t003)                    #doctest:+SKIP
39864
>>> print(tc["030C"])                 #doctest:+SKIP
1206