Stabilize vector renormalisation

[timvisee]

Preprocessing or normalizing vectors should be stable. Meaning that preprocessing or normalizing a second time should give the exact same values.

We need stability because transferring points to another shards with stream records transfer (and SyncPoints) normalizes vectors again. Their values must remain exactly the same.

This PR makes renormalization stable by using 1e-6 as normalization threshold. It adds a test to assert it works as expected for our use case.

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