This is more of a proof-of-concept PR.
It tries to address #1474 by defining a set of core operations a backend should implement to be qibo compliant. Notably, the usual backend.np pattern is removed here. As a consequence, many of the methods from the NumpyBackend are moved to the AbstractBackend as they are completely general and backend agnostic once the core operations are defined.
Actually, it might even be a good idea to move some of them from the backend to some more suitable place. For instance, several methods that implement operations on quantum states might be moved directly to the qibo.result.QuantumState object, similarly for Hamiltonians and others.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.