Collapse reanalysis retrievals into one coverage per point

[andreas-grafberger]

Is your feature request related to a problem? Please describe.

A timeseries retrieval over hdate or date (with a single step) currently returns one coverage per hdate/date, each with a 1-element t-axis.

For the request below, ideally we would get a single PointSeries coverage per point with
t = [hdate + step for each hdate].

{
    "time": "0000",
    "hdate": "19920101/to/19920103",
    "step": 6,
    "feature": {"type": "timeseries", "time_axis": "hdate", "points": [[35.0, -20.0]]}
}

This is closely related to #109 which aims to add support for this in the decoders. This issue here focuses on the encoders and would aim to directly combine multiple dates / hdates into a single coverage.

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