make CsubsetDT callable from C from in other packages

[lsilvest]

Closes #3751.

[jangorecki]

Thanks for PR. I added some documentation. For future PRs please create new branch rather than commiting to master (even if it is just master in your fork). Ideally push it to Rdatatable repo directly, if you are not a member yet @mattdowle should add you soon.

[lsilvest]

Sure, will make sure I create a new branch. Thanks for incorporating this!

[codecov]

Codecov Report

Merging #4084 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4084      +/-   ##
=========================================
+ Coverage    99.4%   99.4%   +<.01%     
=========================================
  Files          72      72              
  Lines       13675   13676       +1     
=========================================
+ Hits        13594   13595       +1     
  Misses         81      81

Impacted Files	Coverage Δ
src/init.c	100% <100%> (ø)	⬆️
src/subset.c	100% <100%> (ø)	⬆️

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[mattdowle]

Thanks, @lsilvest! I invited you to be project member; you'll need to accept the invitation button that Github shows on your profile or repositories page. Once you accept you'll be able to create branches in main project for next time.
I'll do a quick follow up commit after merge to move the news item up and add you to DESCRIPTION contributors.