Sentinel-3: interpret coordinates as pixel center during LUT lookup

[jdries]

Sentinel-3 coordinates are likely to be specified as pixel centre:
Geolocated x (across track) coordinate of detector FOV centre

From:
https://sentiwiki.copernicus.eu/__attachments/1672112/S3IPF.PDS.005.2%20-%20Sentinel%203%20SLSTR%20Product%20Data%20Format%20Specification%20Level%202%20Land%202020%20-%202.9.pdf?inst-v=e49045fb-24ed-4f58-8b10-4339088f6644

This has an impact when doing nearest neighbour lookup here:

openeo-geopyspark-driver/openeogeotrellis/collections/sentinel3.py

Line 603 in 059e2c6

distances, lut_indices = tree.query(target_coordinates, k=1,

Because it seems to be the case that 'target_coordinates' are upper-left corner coordinates. This can cause a shift of half a pixel to occur.