rtable - Delete the empty part of a routing table
Reduce the size of the routing table x. We take the maximal non-empty submatrix.
routing table defined as [length1 path1] [lentgh2 path2] [length3 path3] ... [lentghp pathp] x=[7 1 2 3 4 5 6 7 0 0 0 0 4 5 6 7 8 0 0 0 0 0 0 0 3 1 2 3 4 0 0 0 0 0 0 0 6 3 5 8 1 6 5 0 0 0 0 0 2 5 6 0 0 0 0 0 0 0 0 0] [x]=rtable(x)