Internally compile relative phases to virtual z

[alecandido]

The relative phase and virtual z representation are equivalent, through a simple mapping. However, they might be convenient for different reasons

• the VirtualZ objects avoids explicitly coupling pulses to preceding/subsequent pulses, pushing this to the driver implementation
• the Pulse.relative_phase is convenient to sweep, since it would avoid sweeping two VirtualZ objects

While it can be meaningful keeping both at an interface level, it is inconvenient to require all drivers to implement both.

Furthermore, VirtualZ are more suitable for hardware realization, since they map directly to frame rotations, which can be handled adjusting suitable NCO parameters.

Thus, it would be helpful to process the sequence before passing it down to the drivers, in order to

• convert all .relative_phase != 0 into VirtualZ operations
• propagating all sweepers on relative phases into sweeps of the respective VirtualZ
• possibly merging subsequent VirtualZ into unique once (unless swept by multiple independent sweepers)