About weighted graph support.

Hello, lots of thanks for your great work on graph in js.

Here is question I've encountered in using these awesome projects. I am using `pm` to visualize some GitHub projects networks and it works fine right now but I need link weight support from the layout generation method.

I am using `ngraph.offline.layout` right now and I looked into the projects and I found that actually the physical simulator supports link weight/length in spring force calculation. But as this project is the basic data structure of `ngraph` and the type of `addLink` method only supports `fromId`, `toId` and `data` for custom data store.

Right now I need to do this as a work around:

```
(addLink(fromId, toId) as any).length = l;
```

Should I modify the interface to let users know that link length is supported already?

Thank you for your awesome work again~

Start with the addLink interface in ngraph.graph and trace how link data is exposed to ngraph.offline.layout and its physical simulator. Confirm how link length is used in spring-force calculation, then define and test an explicit supported way to provide link length instead of assigning it through a type escape hatch.

Written by the indexing model from the issue text.