Info.json format changes

Patched to support the following format (with backwards compatibility):

```json
{
  "arguments": {
    "files": {
      "image": "rgb.jpg",
      "composite": "composite.json"
    }
  },
  "metadata": {
    "fields": {
      "A": "temperature"  // RGB images
    },
    "scalars": {
      "B": "x position"  // scalar values stored in RGB
    }
  }
}
```

I think the documentation has the order of these two backwards :smile: