Triad census is defined only for directed networks. The input is always
treated as directed.
MAN notation describes triads by:
M: number of Mutual (reciprocal) edges
A: number of Asymmetric edges
N: number of Null (absent) edges
The 16 triad types are:
003, 012, 102, 021D, 021U, 021C, 111D, 111U,
030T, 030C, 201, 120D, 120U, 120C, 210, 300