def timetree_likelihood(self):
        '''
        Return the likelihood of the data given the current branch length in the tree
        '''
        LH = 0
        for node in self.tree.find_clades(order='preorder'):  # sum the likelihood contributions of all branches
            if node.up is None: # root node
                continue
            LH -= node.branch_length_interpolator(node.branch_length)

        # add the root sequence LH and return
        if self.aln:
            LH += self.gtr.sequence_logLH(self.tree.root.cseq, pattern_multiplicity=self.multiplicity)
        return LH
Return the likelihood of the data given the current branch length in the tree