chore: Fix spelling errors and modernize pointer initialization (#2631)

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 <[email protected]>
Co-authored-by: Pedro Gomes <[email protected]>

Author: shbhmexe

Common/src/CConfig.cpp

@@ -6192,8 +6192,8 @@ void CConfig::SetMarkers(SU2_COMPONENT val_software) {
     }
   }
 
-  /*--- Idenftification fo Giles Markers ---*/
-  // This is seperate from MP and Turbomachinery Markers as all mixing plane markers are Giles,
+  /*--- Identification of Giles Markers ---*/
+  // This is separate from MP and Turbomachinery Markers as all mixing plane markers are Giles,
   // but not all Giles markers are mixing plane
   for (iMarker_CfgFile = 0; iMarker_CfgFile < nMarker_CfgFile; iMarker_CfgFile++) {
     Marker_CfgFile_Giles[iMarker_CfgFile] = NO;
@@ -10017,8 +10017,8 @@ void CConfig::SetProfilingCSV() {
 
   /*--- Allocate and initialize memory ---*/
 
-  double *l_min_red = NULL, *l_max_red = NULL, *l_tot_red = NULL, *l_avg_red = NULL;
-  int *n_calls_red = NULL;
+  double *l_min_red = nullptr, *l_max_red = nullptr, *l_tot_red = nullptr, *l_avg_red = nullptr;
+  int *n_calls_red = nullptr;
   double* l_min = new double[map_size];
   double* l_max = new double[map_size];
   double* l_tot = new double[map_size];

SU2_CFD/include/solvers/CGradientSmoothingSolver.hpp

@@ -275,7 +275,7 @@ class CGradientSmoothingSolver final : public CFEASolverBase {
   }
 
   /*!
-   * \brief Set the current working dimension, if the seperate dimension option is set.
+   * \brief Set the current working dimension, if the separate dimension option is set.
    * \param[in] iDim - the dimension we are currently working in.
    */
   inline void SetCurrentDim(unsigned int iDim) {