Backend in the protocols to reach execute_circuits()

[Juan Cereijo]

It would be nice to have a way to reach the execute_circuits() to unroll circuit routines from the qibolab backend inside the protocols.

[Andrea Pasquale]

For this there should be two solutions:

1. (proper solution): Add backend in the protocols acquisition. Eventually we could replace backend with platform and access the platform using backend.platform.
2. (temporary solution): You can access the backend inside a routine by using GlobalBackend provided by qibo.

[Juan Cereijo]

The 2nd solution works and it would look like what I was doing for #597

[Edoardo Pedicillo]

After the merging of #597, we can close this issue.