Clean up sources

[anderslreed]

Some subclasses of source.Source override modify_request_args to set the provider argument. This can be achieved by setting the api_id of the source in sources.json.

Checking for resource_type != 'data', such as in istat.py is redundant since api.Resource._request_from_args() performs this check later to set provider_id later.

This allows some subclasses of source.Source to be eliminated.