Unable to create an empty Tree

[josselin-lefevre-tfs]

Hi,

I am unable to create an empty Tree despite the fact that the constructor seems to exist on the C++ side:

Higra/include/higra/structure/tree_graph.hpp

Lines 107 to 109 in 3e0d566

	tree() : _root(invalid_index), _num_vertices(0), _num_leaves(0) {
	}

The code below throws a segmentation fault.

import higra as hg

t = hg.Tree([])

This bug was reproduced using the version 0.6.5.