Given a credal partition, we can compute, for each pair of objects, a "pairwise mass function"
on a frame \(\Theta=\{s,\neg s\}\), where \(s\) means that the two objects belong to the same
cluster, and \(\neg s\) is the negation of \(s\). Function pairwise_mass
compute these
pairwise mass functions for all object pairs. The result is return as a list with "dist" objects
containing the masses of each of the two elements of \(\Theta\), and the masses on the empty set.