build.ext.nblist: Internal: generates neighborhood list for diploid loci
Description
This is for use in alleleinit.
Given a neighborhood list of individuals, a new neighborhood list is
generated in which there are two entries for each individual (entry 1
and 2 refer to individual one, 3 and 4 to individual 2 and so
on). Neighborhoods are preserved and additionally the two entries
belonging to the same individual are marked as neighbors.