defaultNodeMass returns NaN

defaultNodeMass returns NaN and *Node mass should be a number* is thrown

https://github.com/anvaka/ngraph.forcelayout/blob/f1c47b57035869750a9bfbe9336dee63eba1e90d/index.js#L388

seems that *links* does not have *length* but *size*, as newer **graph.getLinks** does not return *array* anymore

as a temporary workaround **nodeMass** function in physicsSettings helps..

```
nodeMass : (nodeId) => {
const links = graph.getLinks(nodeId);
if (links && links.length) return 1 + links.length / 3.0;
else if (links && links.size) return 1 + links.size / 3.0;
else return 1
}
```

Start at index.js line 388, where defaultNodeMass uses graph.getLinks(nodeId), and compare the returned object's size behavior with the issue's nodeMass workaround. Confirm that the default mass is numeric for graphs whose links collection has size rather than length, and verify that the *Node mass should be a number* error no longer occurs.

Written by the indexing model from the issue text.