Plugwise-smile

- Add a service to dismiss/delete the Plugwise Notification(s) from within HA Core (plugwise.delete_notification)
- Improve detection of switch-groups and add group switching for the Stretch

- Improve error handling, add group switching of plugs (Adam).

- Initial support for Stretch (v2/v3) including tests
- Force gzip encoding, work-around for aiohttp-error
- Improve P1 DSMR legacy support
- Ensure `gateway_id` is properly defined (i.e. not `None`)
- b4: Use `domain_objects` over `direct_objects` endpoints
- Remove py3x internal modules (as requested per 86)
- CI-handling improvements and both 3.7 and 3.8 testing
- Code cleanup and output formatting improvements