Specify how acquisitions map to the independent variables (e.g., acq_ch, acq_idx, ).

Old-title: Data registers, control registers, and return values

Creating this issue to track the discussion on data registers and return types.

In #36 (closed) , we have defined acquisition protocols. An acquisition protocol specifies how to interpret an acquired signal.

Acquisition protocols (when performed) return data. This data needs to be stored somewhere. Here we discuss how to specify where to store this, and how this should be retrieved. This effectively corresponds to determining the return type of a "schedule" when executed.

Relates to !51 (merged)