XP-544 fix aggregate module

[janpetschexain]

References

• XP-544

Summary

• handle division by zero in WeightedAverageAggregator
• switch to new nomenclature according to the glossary and removed outdated types
• split weight_update argument into its proper arguments model_weights and aggregation_data

Are there any open tasks/blockers for the ticket?

Follow up:

• XP-547 write tests for aggregate.py

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Merge request checklist

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Code checklist

• Conforms to the branch naming XP-XXX-<a_small_stub>.
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• Added or updated tests if needed.
• Added or updated code documentation if needed.
• Conforms to Google docstring style.
• Conforms to XAIN structlog style.