AdjacencyGraph.cs

* This check was incorrectly implemented in MatrixMarketReader and was
  now moved to AdjacencyGraph's AddElement
* It remains unclear what should happen if we pass a diagonal matrix
  such as bcsstm01 to the algorithm. Our current implementation produced
  empty compressed and seed matrices which feels incorrect.

  I feel like the seed matrix should in this case be the identity matrix
  but I am unsure if it is as simple as providing the seed matrix if the
  graph is empty (only diagonal elements). Because if there is one
  non-diagonal element the diagonal would nonetheless be neglected in
  calculating the seed matrix.