structural_balance : plots:  definition C and D: including 1988, 1990
export_data: differentiating between Nan and missing data point
structural_balance : plots: definition_C : if (A,B) pair doesn't show up in dataset file, it is considered missing point. Representing this as NO_LINK in definition_C
structural_balance : plots: definitions: updated logging, passing in right threshold for defn_C for log statements
structural_balance : plots: definitions: definition_C becomes def_C1, def_C2 variant documented