Fix handling of weighted metabolic graph as undirected

The "AsUndirected" processing of BioGraph simply add reverse edges for all existing edges. However, in the case of weighted graphs, edges' weight wasn't passed to the newly created ones. This raised other concerns, such as the use of node weighting transferred to edges (such as degree weighting used in distance matrices), where reversed and original edges sharing the same weight might not be suited despite ensuring the symmetry property of the metric space. There is also special cases that might be of interest for some apps, such as avoiding doubling of reversible reaction edges in compound graphs. This merge request aims at ensuring appropriate behaviour of "undirecting" BioGraphs and expand test cases