Allow for customArgs

Add custom args so that custom comparison methods can provide more args such as max etc.

Fields would change to.

```js
var fields = [
    { name: "x", measure: nn.comparisonMethods.custom, args: {max: 10} },
    { name: "y", measure: nn.comparisonMethods.custom, args: {max: 10} }
];
```

custom comparison mathods would be
```js
nn.comparisonMethods.custom = function(a, b, args) {
    // Test for args and enforce defaults...

    // compare something...
    var value = ...
    return value; // between 0 and 1
};
```