uint Tree::num_nodes() const;
a uint: the numbef of nodes in the tree, 
including tips, internal nodes and the root.
Other public methods in SPLITT::Tree: SPLITT::Tree::BranchLengths,
  SPLITT::Tree::FindChildren,
  SPLITT::Tree::FindIdOfNode,
  SPLITT::Tree::FindIdOfParent,
  SPLITT::Tree::FindNodeWithId,
  SPLITT::Tree::HasBranchLengths,
  SPLITT::Tree::LengthOfBranch,
  SPLITT::Tree::OrderNodesPosType,
  SPLITT::Tree::OrderNodes,
  SPLITT::Tree::SetBranchLengths,
  SPLITT::Tree::SetLengthOfBranch,
  SPLITT::Tree::Tree,
  SPLITT::Tree::num_tips