CObj: add optional warning if method returning a <: CObject doesn't use an implicit wrapper

As a best practice, methods returning a wrapped C object should always take an implicit wrapper, i.e.
```scala
def getFoo()(implicit w: CObjectWrapper[Foo]): Foo = extern
```
instead of
```scala
def getFoo(): Foo = extern
```

To help enforce this policy, it should be possible to activate warnings/errors if it is broken.