[2531] Require geoinfo_channels for obs streams

[vhertel]

Description

When a stream config does not specify geoinfo_channels, data_reader_obs.py falls back to inferring them by position: it takes the columns between lon and the first obsvalue column (line 90). This only matches if the Zarr is laid out with an order of [coords][geoinfo][obsvalue], which is not consistent with some obs datasets.

This PR removes the positional inference and instead requires geoinfo_channels to be set explicitly.

Issue Number

Closes #2531

Checklist before asking for review

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