loopcount for DiGraph does not work

The following code to obtain the loop count of a digraph fails. none of the suggestions from the error words. the FGraph version works fine.

```
let mutable lg: DiGraph =
DiGraph.createFromEdges [| (0, 2, 1.0); (0, 1, 1.0)|]
|> DiGraph.addEdge (1, 2, 1.0)
|> DiGraph.addEdge (0, 1, 1.0)

Measures.Loop.loopCount lg
```

> typecheck error Value restriction. The value 'it' has been inferred to have generic type
> val it: '_a
> Either define 'it' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.