Byzantine robust aggregator

[s314cy]

Following the aggregator refactor in #578, byzantine-robust aggregation in the federated setting does not work anymore as it remains to be re-implemented with the new architecture in mind.

[JulienVig]

The work pre-refactoring has been left in the code unused, we are removing it in #652. For future use, it can be found in discojs/discojs-core/src/aggregator/robust.ts at commit b9db50f

[tharvik]

• the initial implementation was done in Adding byzantine-tolerant aggregator #489
• based on a MLO paper
• byzantine robust in this case is robustness against "bad weights"
• seems quite douable (added to v4 milestone)

thanks @JulienVig for the code de-zombification