modified pbdsim2 for linked incipient

pbdsim2 now takes into account incipient species during death of good
species. One of the incipient species linked to the dead good species is
randomly made good, and all others are linked to that one as a new
"effective parent" (column 6). Additionally, a new incipient from
incipient has its "effective parent" as the good parent of the incipient
parent.
Allows speciation to depend only on close species, rather than some sort
of intrinsic likelihood of extinction.