Two qubit alignment

[alecandido]

Closes #976

I implemented the automated padding on the drive channel. Currently, it's undocumented. However, we should maybe dedicate some more efforts to the compiler, and my proposal is (for the time being) to not even make it part of the public API (cf. #790). So, this should correspond to the expected behavior, and the former one was classifiable as a bug. Thus, we could say the compiler knows what to do, but (for the time being) how it is doing that is an internal detail. As soon as we'll take the time to review it (with no rush), we could decide to publicly expose, and properly document what is doing.