fix several const qualifier issues: double ones, meaningless ones, some missing ones, etc.

(note that const qualifiers are set by internall::nested)

Eigen/src/SparseCore/SparseView.h

@@ -61,7 +61,7 @@ public:
   inline Index outerSize() const { return m_matrix.outerSize(); }
 
 protected:
-  const MatrixTypeNested m_matrix;
+  MatrixTypeNested m_matrix;
   Scalar m_reference;
   typename NumTraits<Scalar>::Real m_epsilon;
 };