Add rulers to change D3 parameters: Charge, gravity, link distance etc...

[rchateauneu]

At the moment, there is no way to change the D3 display parameters of force layout, such as gravity, charge, link distance. It would be helpful to add a ruler to display and change these parameters and see the effect on display.

It is especially convenient when many nodes are present, making the display confusing.

Maybe these parameters could be edited per node class.