Single shot acquisition mode

[alecandido]

The single shot averaging mode is alternative to the cyclic and sequential, but it is actually playing on a different dimension.
Specifically, the two averaging modes are specifying the position of the shots loop (as innermost or outermost), while the single shot mode is just telling not to average - but it says nothing about the loop position.

I'm assuming that AveragingMode.SINGLESHOT is actually equivalent to "no average + cyclic", placing the loop outermost in the Qblox driver (since looping the shots as the last iteration should be almost always advisable).
However, in principle, even the "no average + sequential" may be an available alternative. I assume there is no interest in that, and that we don't want to implement it. But we may want to document the exact meaning of the existing option (to avoid ambiguities).

(mainly for @stavros11, to sync the drivers - but opinions are welcome also from the Qibocal crowd, i.e. @andrea-pasquale @Edoardo-Pedicillo - and whoever in general)