[Restrictions] Functions should have a type

In this example:

```
abs(train.specs.maxLoad) > 0
```

It's impossible to translate this example to JPA Criteria API considering the function has to define its type:

```
   <T> Expression<T> function(String name, Class<T> type,
Expression<?>... args);
```

I think the solution would be to add `type` to `FunctionExpression`.


