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 https://github.com/higra/Higra/blob/3e0d566becadc86d9244ed2e67ffc35a041ba0ec/higra/image/graph_image.py#L30-L44