Create record pairs from blocks of ids.
makeBlockingPairs(id_vec)A list of integer vectors.
A matrix with two columns, each row holding the ids of one record pair.
Each element in id_vec holds the ids of a set of records which agree
for a particular blocking variable (see compare.dedup for the
blocking mechanism). For each block, all unordered pairs of the ids are
created and concatenated in the output.