Add to edges with no node check

[andrea-cassioli-maersk]

Is your feature request related to a problem? Please describe. Running some profiling on our application, it seems that adding edges to a bidirectional graph is quite expensive, and in particular quite an amount of time is spent in looking up if origin and destination nodes are valid, i.e. they exists.

Describe the solution you'd like I was thinking about a flag to disable node existence checking. It could be enable by default, without breaking current interface.