Support for MultiPointSeries CoverageJSON domain types

[andreas-grafberger]

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

A timeseries feature with N points currently returns N (x dates x levels x numbers) separate
PointSeries coverages, each repeating mars:metadata and the domain scaffolding. For thousands
of stations over years this is both large and slow to post-process.

The spec defines MultiPointSeries for exactly this case.

Desired output (combined with point 2):

{
    "type": "CoverageCollection",
    "domainType": "MultiPointSeries",
    "coverages": [{
        "type": "Coverage",
        "domain": {
            "type": "Domain",
            "axes": {
                "t": {"values": ["1992-01-01T06:00:00Z", "1992-01-02T06:00:00Z", "1992-01-03T06:00:00Z"]},
                "composite": {
                    "dataType": "tuple",
                    "coordinates": ["x", "y"],
                    "values": [[-20.0, 35.0], [-22.0, 38.0]]
                }
            }
        },
        "ranges": {
            "dis06": {
                "type": "NdArray", "dataType": "float",
                "axisNames": ["t", "composite"], "shape": [3, 2], "values": [...]
            }
        }
    }]
}

Note that composite tuples are [x, y] = [lon, lat] per spec. We would need to implement this on the encoder and decoder-side. Going from a MultiPointSeries to an xarray dataset in the decoder would enable us to easily return all requested data in a single dataset, rather than a list of multiple small datasets.