json: references to nodes

As it turns out, all references to other nodes in the json file has to be in uri format.

For example, this is not prov-json compliant:
```
"e10" : {
    "prov:activity" : "p6",
    "prov:entity" : "d3"
}
```

But this is compliant:
```
"e10" : {
    "prov:activity" : "http://rdatatracker.org/p6",
    "prov:entity" : "http://rdatatracker.org/d3"
}
```

What would the uri be?