add missing aggregator code documentation

[ghubertpalo]

Add missing code documentation in the aggregator library.

[github-actions[bot]]

Unit Test Results

    7 files  ±0    22 suites  ±0   1m 26s :stopwatch: -2s 293 tests ±0  293 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  294 runs  ±0  294 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 27d793d7. ± Comparison against base commit e9993df8.

:recycle: This comment has been updated with latest results.