Annotate data from multiple devices

[PeterKraus]

Currently, when the partial Datasets generated by separate devices are concatenated into one using xarray.concat(), we align the dataset on the "uts" coordinate, but the data_vars are unmodified. This means that if multiple devices in a pipeline produce the same column (e.g. "flow" for a flow meter), its difficult to disambiguate.

Solution: the role of the device in the pipeline should be prepended to all columns.