SF: Better discrimination of arrays and datasets

[MichaelHuth]

With the introduction of averaging over groups of datasets we have the situation that we need to evaluate arrays of datasets in SF.

Currently we only discriminate arrays from datasets by the type of the wave stored. Where waves that are not numeric or text are treated as dataset (only waveref type atm). However arrays of datasets are also waveref waves.

To discriminate these one solution is to tag waves that are datasets through the JSON wavenote, such that we can apply different code paths in handling these.

And we should also tag helper text waves that resolve permanent waves.