Access and output of sequences from the 'sw1d_channel' instance

[unitedoffice]

It would be useful if after the simulation, the simulated sequences (e.g. discharges) of each element of the 'sw1d_channel' instance could be accessed and saved, similar to how it is possible for elements of the land models. So far, only NaN values are being stored. Code example:
for element in hp.elements.search_keywords("LIAS"): element.model.sequences.fluxes["discharges"].save_series()

[tyralla]

Yes, this behaviour is logical if one knows the details of HydPy-SW1D and the collective concept, but surprising for those who think that sw1d_channel just works like "normal" models.

We will need to discuss whether we prefer to send the date to the sequences of sw1d_channel somehow (with some additional computational cost), or if we prefer to deny access to these sequences, remove them, emit warnings, or so.