[Math] Improve sparse matrix loop example

The current example only works inside the class, not for user code.

Author: ferdymercury

math/matrix/src/TMatrixTSparse.cxx

@@ -31,14 +31,17 @@
  As an example how to access all sparse data elements:
 
 ~~~
- for (Int_t irow = 0; irow < this->fNrows; irow++) {
-   const Int_t sIndex = fRowIndex[irow];
-   const Int_t eIndex = fRowIndex[irow+1];
+ TMatrixTSparse<double> M;
+ auto rowIndex = M.GetRowIndexArray();
+ auto colIndex = M.GetColIndexArray();
+ auto matrix = M.GetMatrixArray();
+ for (Int_t irow = 0; irow < M.GetNrows(); irow++) {
+   const Int_t sIndex = rowIndex[irow];
+   const Int_t eIndex = rowIndex[irow+1];
    for (Int_t index = sIndex; index < eIndex; index++) {
-     const Int_t icol = fColIndex[index];
-     const Element data = fElements[index];
-     printf("data(%d,%d) = %.4e\n",irow+this->fRowLwb,icol+
-                                               this->fColLwb,data);
+     const Int_t icol = colIndex[index];
+     const double data = matrix[index];
+     printf("data(%d,%d) = %.4e\n", irow + M.GetRowLwb(), icol +  M.GetColLwb(), data);
    }
  }
 ~~~