select codon columns of alignments (of size 0 mod 3) in orthomam that
are quasi full, then randomly select n of this set. By "quasi full", I
mean columns that have a non-stop codon for all species but a few. For
"a few = 0", that led to about 3000 sites, "a few = 1" about 25000 and
"a few = 2" about 90000.