Fixed error in Node.setChild().  Closes #544.
Fixed error in Node.setChild().  Closes #544.

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