fixed python3 specific bug involving ".."

fixed a python3-specific bug in ttree.py causing expressions containing ".." (like "@atom:../A") to sometimes cause an uncaught exception. Example:

MoleculeTypeA {
  write_once("In Settings") {
    @atom:../A
  }
}
MoleculeTypeB {
}

These kinds of expressions should work again now. (The A leaf node should be created in "/" now.)