Add option to use a preexisting graph in graph_4_adjacency_2_khalimsky

[PerretB]

The function graph_4_adjacency_2_khalimsky always allocate a new graph which might be inefficient for certain use case where the graph could be reused

Higra/higra/image/graph_image.py

Lines 30 to 44 in 3e0d566

	def khalimsky_2_graph_4_adjacency(khalimsky, extra_border=False):
	"""
	Create a 4 adjacency edge-weighted graph from a contour image in the Khalimsky grid.
	:param khalimsky: a 2d array
	:param extra_border: if False the shape of the Khalimsky image is 2 * shape - 1 and 2 * shape + 1 otherwise, where shape is the shape of the resulting grid graph
	:return: a graph (Concept :class:`~higra.CptGridGraph`) and its edge weights
	"""
	graph, embedding, edge_weights = hg.cpp._khalimsky_2_graph_4_adjacency(khalimsky, extra_border)
	hg.CptGridGraph.link(graph, hg.normalize_shape(embedding.shape()))
	hg.set_attribute(graph, "no_border_vertex_out_degree", 4)
	return graph, edge_weights