Eli Arbel

PR #1196 should suffice, thanks a lot

@ajavadia I think that quantum circuit white listing should eventually be based on the `Operation` class, i.e. `QuantumCircuit` can only append objects of classes derived from `Operation`

Perhaps I'm missing here something but it seems to me that slots and mixin are orthogonal, at least in our discussion here. The use if mixin here is to signify...

I assume this should remain open for the other tasks to be completed

> As for `ExperimentData.load(exp)`, I think this overloading makes the already confusing interface even more confusing Good point, I tend to agree with keeping the function taking `exp_id` only