Refactor ALL AiiDA interactions to an AiiDAService

[edan-bainglass]

Currently, AiiDA interactions are spread across the codebase. I propose we move all such interactions to a dedicated AiiDAService class. Ideally, all from aiida import ... should be moved here, as well as any method that makes a call to AiiDA.

Two benefits:

1. Maintenance - any change in AiiDA would require only changes in this class
2. Future abstraction - we could then more easily switch the backend associated with the service from DB to REST API, as needed