Error: Node mass should be a number

Hi all,

when using this module as part of layouting the graph via

```
console.log('Loading graph');
var graph = require('./loadgraph.js')();
console.log('Loaded ' + graph.getNodesCount() + ' nodes; ' + graph.getLinksCount() + ' edges');

var createLayout = require('ngraph.offline.layout');
var layout = createLayout(graph);
console.log('Starting layout');
layout.run();
```

I get following error:

```
Loading graph
Loaded 79566 nodes; 255149 edges
/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:357
throw new Error('Node mass should be a number')
^

Error: Node mass should be a number
at updateBodyMass (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:357:13)
at initBody (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:290:7)
at /Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:265:7
at Object.forEachNode (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.graph/index.js:543:11)
at initPhysics (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:264:11)
at Function.createLayout [as get2dLayout] (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout/index.js:35:3)
at createLayout (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.forcelayout3d/index.js:21:23)
at createLayout (/Users/aydin/Workspace/ngraph_native_suite/node_modules/ngraph.offline.layout/index.js:19:16)
at Object. (/Users/aydin/Workspace/ngraph_native_suite/bin/layout.js:6:14)
at Module._compile (internal/modules/cjs/loader.js:1085:14)
```

Any idea how to fix this? This does not happen with ngraph-binary layouting (which had another unrelated sideffects in the result)

Reproduce the layout from bin/layout.js using the graph-loading sequence shown, then inspect ngraph.forcelayout/index.js around updateBodyMass at line 357 and the initBody call stack. Done means layout.run() completes without the Node mass error; the issue also notes comparing behavior with ngraph-binary.

Written by the indexing model from the issue text.