Decide profile handling

[ltalirz]

To keep things simple, the aiida-restapi package started without explicit handling of AiiDA profiles (it simply uses the default profile of the AiiDA installation where it is running).

This is fine for the moment, but will likely need to be generalized once this API starts being used in production.
Note that the decision of how to implement profile handling may depend on how we integrate authentication with the AiiDA side #5 .