big reorganisation of asserts, so that:

0) asserts are only done in the public API, except for a few ones explicitly
   named eigen_internal_assert.
1) internal asserts are disabled unless EIGEN_INTERNAL_DEBUGGING is defined.
   This limits the impact of debugging on performance.
2) no 'unused argument' warnings anymore when compiling with -DNDEBUG

src/Core/Row.h

@@ -40,7 +40,7 @@ template<typename MatrixType> class Row
     Row(const MatRef& matrix, int row)
       : m_matrix(matrix), m_row(row)
     {
-      EIGEN_CHECK_ROW_RANGE(matrix, row);
+      assert(row >= 0 && row < matrix.rows());
     }
     
     Row(const Row& other)