export merge.data.table method

[jangorecki]

closes #2618

[codecov]

Codecov Report

Merging #3588 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3588      +/-   ##
==========================================
+ Coverage   97.63%   97.63%   +<.01%     
==========================================
  Files          66       66              
  Lines       12702    12706       +4     
==========================================
+ Hits        12401    12405       +4     
  Misses        301      301

Impacted Files	Coverage Δ
src/froll.c	100% <0%> (ø)	⬆️
src/frolladaptive.c	100% <0%> (ø)	⬆️

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