suppress printing of node names in Newick output

fix bug in Newick parser

filter for si2r

SEIR sampler now supports destructive sampling

new BDEI and BDSS models

fix bug

update vignettes

make SI2R sampling destructive

alternative filter scheme for SEIRS

- the parser can now handle metadata in branch-length strings

- newick() and yaml() now report branch-lengths and times with greater precision