Workbench: use the latest model spec structures

[davemfish]

When we redesigned the model spec, we avoided modifying the Workbench by translating the new spec back to the same structure the Workbench always used. It looks like we do this in ModelSpec.to_json:

# rename 'inputs' to 'args' to stay consistent with the old api
spec_dict.pop('inputs')
spec_dict['args'] = {_input.id: _input for _input in self.inputs}

It would be nice to update the Workbench so it can use inputs directly.