affiliation network class

[corybrunson]

Introduce a subclass affnet of igraph and use it as a method for most functions in this package.

[corybrunson]

Not sure if this is necessary. Here are the functions that could be given an affnet method:

• transitivity() (alias of triad_closure())
• triad_census()
• prettify() (probably this function should be renamed anyway)

[corybrunson]

Currently triad_census() and triad_closure() pass non–affiliation network graphs to igraph::triad_census() and transitivity(). This is probably preferable to introducing a new object class, since (a) bitriad depends on igraph and (b) it may be useful in specific circumstances to ignore affiliation network structure.