Breadth first search subgraph

I was looking for a simple breath first search implementation and was surprised that there does not seem to be one.

Maybe it makes sense to add one?

Here my code:

```typescript
function getSubGraph(graph: Graph, start: NodeId): Graph {
let subgraph = createGraph();

if (!graph.hasNode(start)) return subgraph;
const startNode = graph.getNode(start)!;

let queue = [startNode];
subgraph.addNode(start, startNode.data);

while (queue.length > 0) {
let currentNode = queue.shift()!;

for (const link of currentNode.links) {
const visitedNode = graph.getNode(link.toId)!;
if (!subgraph.hasNode(visitedNode.id)) {
subgraph.addNode(visitedNode.id, visitedNode.data);
subgraph.addLink(link.fromId, link.toId, link.data);
queue.push(visitedNode);
}
}
}
return subgraph;
}
```

Start by locating the subgraph entry point and the Graph, NodeId, createGraph, and getSubGraph APIs in ngraph.subgraph. Use the supplied breadth-first traversal as the behavioral reference, including an empty result for a missing start node; done means reachable nodes and links are returned in a subgraph.

Written by the indexing model from the issue text.