fix treeparser for trees with single child nodes
fix treeparser for trees with single child nodes

CompEvol/beast2,tgvaughan/beast2,CompEvol/beast2,CompEvol/beast2,Anaphory/beast2,tgvaughan/beast2,tgvaughan/beast2,tgvaughan/beast2,Anaphory/beast2,CompEvol/beast2,Anaphory/beast2,Anaphory/beast2