Potential problems with `Graph.weighted_outdegrees`

[M-Lampert]

While fixing an issue with the Graph.weighted_outdegrees()-function (#253), I found some inconsistencies that we should probably be addressed:

• Why is there a function for computing the weighted outdegrees, but not the indegrees?
• Here we hardcoded edge_weight as the attribute that contains the weights of the edges, however, in other functions such as Graph.laplacian(), you can pass a name for that attribute using the parameter egde_attr. We should make this consistent.
• Is it necessary to have this as an extra function or could we integrate this into Graph.degrees(), using an optional parameter?
  Depending on how we decide to answer the above questions, this might break the current API, so we should this discuss this before implementing any solution.