division by zero for zero distances in tree

[ivanerill]

If TF is present also as target species, the tree may contain zero-length branches, that may cause issues (division-by-zero error) during the phylogenetic weighting process.

[ivanerill]

e.g.

~ INFO     Creating genome: Rhodanobacter_denitrificans_strain_2APBS1 [u'NC_020541.1']
~ INFO     Finished: create genomes
~ INFO     Identifying TF instance for (Rhodanobacter_sp__FW510_R12)
~ INFO     KZC39902.1
~ INFO     Identifying TF instance for (Rhodanobacter_sp__FW510_R10)
~ INFO     WP_082824058.1
~ INFO     Identifying TF instance for (Rhodanobacter_denitrificans_strain_2APBS1)
~ INFO     WP_015447723.1

Tree(rooted=True)
    Clade(branch_length=0, name='Root')
        Clade(branch_length=0.00231481481481, name='Inner1')
            Clade(branch_length=0.1875, name='Rhodanobacter_sp__FW510_R10')
            Clade(branch_length=0.127314814815, name='Inner2')
                Clade(branch_length=0.0, name='Rhodanobacter_denitrificans_strain_2APBS1')
                Clade(branch_length=0.0, name='LexA_Rhodanobacter_denitrificans')
        Clade(branch_length=0.189814814815, name='Rhodanobacter_sp__FW510_R12')