Move xr_reproject from odc-tools to datacube-core

[uchchwhash]

Expected behaviour

Reprojecting data is a fundamental GIS operation and is at the heart of what datacube does, so it's only fitting that we have this functionality as a library function.

Actual behaviour

The current implementation still has not left the prototype package despite being widely used and thoroughly tested.

[snowman2]

I would prefer to see this: opendatacube/odc-tools#58 🤞

[uchchwhash]

You feel rather strongly about this one, don't you @snowman2? Totally agreed.

[stale]

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