Recover ability to set all internal branch lengths equal to 1, accounting for missing internal nodes

[dkoslicki]

Due to changes in 428cd05, the old default of setting all branch lengths to 1 resulted in things like the following. For this profile:

# Taxonomic Profiling Output
@SampleID:CAMI_LOW_S001
@Version:0.9.1
@Ranks:superkingdom|phylum|class|order|family|genus|species|strain
@@TAXID	RANK	TAXPATH	TAXPATHSN	PERCENTAGE
<snip>
0	strain	5||||0	Viruses||||Viruses strain	10

if all internal branch lengths are 1, we should get a distance of 4 between nodes 5 and 0 (due to the missing internal ranks), but now after 428cd05, this will be set to:

• 1 if branch_length_fun=lambda x: 1
• 5 if branch_length_fun=lambda x: x

and so a new option will need to be provided to recover the old behavior of setting all internal branch lengths to 1.