Remove instrument interaction from the Platform class

[AlbertMitjans]

Right now the Platform class contains methods like connect, initial_setup and turn_on_instruments, which act on all instruments of the platform.

Ideally we'd like to use only the instruments needed to execute the given circuit for several reasons, mainly to have more control of what is going on in the lab.

We could move all these methods inside the ExecutionManager class, such that internally it only interacts with the buses needed to execute the experiment.