Update lattigo to 6.2.0

https://github.com/tuneinsight/lattigo/releases/tag/v6.2.0

The main notable update is that we can use the evaluator/encoder/etc structures concurrently, which means we can add a layer of goroutine thread parallelism on top of the lattigo API.

Start with the Lattigo v6.2.0 release linked in the issue and trace the repository's current Lattigo dependency and uses of its evaluator and encoder structures. The work is done when the project uses version 6.2.0 and the relevant Lattigo API can support the described goroutine-level parallelism.

Written by the indexing model from the issue text.