When collapsing branches of length None, don't turn them into zeroes.
Use None as the default branch_length instead of 1.0, and don't write 1.0 or 0.0 branch lengths in Newick if they aren't present.
Don't write None branch lengths in NeXML.
Merge branch 'nexus'
Merge branch 'master' of github.com:bendmorris/biopython