Fix 223 issue with add_coverage

[seabbs]

When by is not null we saw an issue with add_coverage highlighted in #223. This appears to be caused by an issue in the data.table code. Dropping into debug changing the call to the mean function to be explict (i.e base::mean) fixes the issue on my system. If this also causes no reversions on CI etc I'd suggest merging as aa bug fix for bleeding edge users. I can't identify any changes in data.table that would lead to this change so still hypothetical that this was the system level difference.

[codecov]

Codecov Report

Merging #234 (ba57306) into master (b0cee59) will not change coverage.
The diff coverage is 100.00%.

❗ Current head ba57306 differs from pull request most recent head 4952d7e. Consider uploading reports for the commit 4952d7e to get more accurate results

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   91.21%   91.21%           
=======================================
  Files          22       22           
  Lines        1355     1355           
=======================================
  Hits         1236     1236           
  Misses        119      119           

Impacted Files	Coverage Δ
R/summarise_scores.R	89.04% <100.00%> (ø)

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[nikosbosse]

Very confusing... Thanks for looking into this!
I'll just merge it and hope that solves the issue...