dflow exports a fun function ☺

It has the following signature.

```
/**
* Create a dflow function.
*
* @param {Object} graph to be executed
* @param {Object} [additionalFunctions] is a collection of functions
*
* @returns {Function} dflowFun that executes the given graph
*/
```

`dflow.fun(graph)`

returns a *dflowFun* function which has also a *graph* property, to remember that it is a function generated by *dflow*.

Actually *dflow* exposes also the following functions, available only inside a *dflow* graph: