igraph 0.9.8

Fixed

• Assertion failure in igraph_bfs() when an empty roots or restricted vector was provided.
• igraph_diversity() now returns 0 for degree-1 vertices. Previously it incorrectly returned NaN or +-Inf depending on roundoff errors.
• igraph_community_walktrap() does not crash any more when provided with
  modularity=NULL and membership=NULL.

Other

• Documentation improvements.