Structured Gettables that return an xarray dataset
Currently, a Gettable returns an array of data, the associated metadata (units labels etc.) are given as attributes for the class. It has been identified that for certain use cases (e.g., a VNA measurement, quantify-scheduler schedule execution, etc.), supporting returning (xarray) datasets that include the metadata solves a lot of problems.
This change will likely propagate into the measurement control which will need to support a backwards compatibility mode for old-style gettables.