Obscure error if assign is used instead of an annotation.

[AdamHillier]

If an assignment is used instead of an annotation for a nested component there is an obscure error.

E.g. if a user defines a model as:

class FancyModel(Model):
    dataset = Dataset
    ...

and runs configure, either directly or through the CLI, the following error is produced: TypeError: configure() missing 1 required positional argument: 'conf'.

The correct line should either be dataset: Dataset or dataset = Dataset(...).

We should detect this mistake and print a warning.

[AdamHillier]

Closed by #109