Fix #7354, Implementing NSE in cube

[sisyphuswastaken]

Pertains to issue #7354.
Changes implemented:

• isolated logic for processing SDcols from the [.data.table function, encapsulated it into a helper function .processSDcols.
• Added tests for code coverage
• removed the try-catch error block, to make errors more specific and debugging easier
  This PR continues work from Implementing NSE in cube #7540 and and Implementing NSE in cube #7543.

[codecov]

Codecov Report

❌ Patch coverage is 96.36364% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.99%. Comparing base (0216983) to head (2a15aef).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
R/data.table.R	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7603      +/-   ##
==========================================
- Coverage   99.00%   98.99%   -0.01%     
==========================================
  Files          87       87              
  Lines       16893    16935      +42     
==========================================
+ Hits        16725    16765      +40     
- Misses        168      170       +2     

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[jangorecki]

Please use meaningful title for your PR

[sisyphuswastaken]

Sure, I have modified the title to be more specific to the issue