Specify which column name clashes with metrics

[Bisaloo]

Fix #238

[codecov]

Codecov Report

Merging #239 (63b45a9) into master (f356634) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
+ Coverage   91.21%   91.25%   +0.03%     
==========================================
  Files          22       22              
  Lines        1355     1361       +6     
==========================================
+ Hits         1236     1242       +6     
  Misses        119      119              

Impacted Files	Coverage Δ
R/check_forecasts.R	87.38% <100.00%> (+0.72%)	⬆️

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[seabbs]

LGTM to me aside from the tiny linting issues.

[nikosbosse]

Very nice, thanks a lot!