Add force scaling parameter to spring layout.

Add parameter k to specify optimal distance betweneen nodes in spring layout.
The default is 1/sqrt(number of nodes).
Rescale layout on every step to be in [0,1] box.
Remove spurious print and empty notes field.
Fix bug in average weighted connectivity for digraphs when using in-degree.

Fixes #627.
Add test.
Merge pull request #818 from hagberg/627-connectivity

Fix bug in average weighted connectivity for when using in-degree.  Fixes #627
Merge pull request #815 from hagberg/spring-layout

Add force scaling parameter to spring layout.