Function SCIPdigraphAddArc 

pub unsafe extern "C" fn SCIPdigraphAddArc(
    digraph: *mut SCIP_DIGRAPH,
    startnode: c_int,
    endnode: c_int,
    data: *mut c_void,
) -> SCIP_RETCODE

add (directed) arc and a related data to the directed graph structure

@note if the arc is already contained, it is added a second time