Fix #7354 - Implementing NSE in cube

[sisyphuswastaken]

This PR continues work from #7543

I have worked towards encapsulating the SDcols processing logic into a helper function, .processSDcols. In accordance with the previous review, the function has been moved to data.table.R.

I am still working on removing the try-catch block and replacing it with errors for easier debugging.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.91%. Comparing base (851467f) to head (f2f2d9f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
R/data.table.R	91.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7569      +/-   ##
==========================================
- Coverage   98.96%   98.91%   -0.05%     
==========================================
  Files          87       87              
  Lines       16734    16800      +66     
==========================================
+ Hits        16560    16618      +58     
- Misses        174      182       +8     

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