Convert a matrix denoting network connectivity (edges or flows) to a list
denoting edges
Parameters:
edge_matrix:array
where rows denote network edge origins, columns denote
network edge destinations, and non-zero entries denote the
existence of an edge between a given origin and destination
Returns:
edge_list:list
of tuples where each tuple is of the form (o,d) where o is an
origin id and d is a destination id