[claxn]

This PR adds all STAC band statistics (https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#statistics-object) to each array (native and multiscaled). Statistics are added to each asset under the stac_discovery attribute. All statistics are computed using encoded values. Here is an example:

......attrs["stac_discovery"]["assets"]["b05_60m"])

{'href': '/measurements/reflectance/r60m/b05',
 'statistics': {'count': 701942,
                'maximum': 17815,
                'mean': 5717.244802277111,
                'minimum': 112,
                'stddev': 3977.472783907385,
                'valid_percent': 20.960375048523396},
 'title': 'b05_60m'}

The latest sample data for Sentinel-2 is located here: s3://continuous-integration/eopf-geozarr/branches/feat-stac-band-statistics.

As a next step, the statistics need to be interpreted correctly by eopf-stac to map the statistics as @lhoupert describes it here (I will contact Mario Winkler and also share the sample data with him).