This PR improves code quality through two types of fixes:

Spelling Corrections (3 fixes):
- CConfig.cpp (L6169): "Idenftification fo" → "Identification of"
- CConfig.cpp (L6170): "seperate" → "separate"
- CGradientSmoothingSolver.hpp (L278): "seperate" → "separate"

Code Modernization (1 fix):
- CConfig.cpp (L9991-9992): Replace NULL with nullptr for C++11 compliance and better type safety

All changes are safe, non-breaking, and improve code quality.

Signed-off-by: shbhmexe <shubhushukla586@gmail.com>