Update HessianFactor.cpp

Fix JacobianFactor and HessianFactor on platforms where Eigen::Index is not the same size as gtsam::Key

Eigen::Index is defined as std::ptrdiff_t (size_t), which is not always the same size as gtsam::Key (uint64)

gtsam/linear/HessianFactor.cpp

@@ -38,7 +38,7 @@ using namespace std;
 namespace gtsam {
 
 // Typedefs used in constructors below.
-using Dims = std::vector<Eigen::Index>;
+using Dims = std::vector<Key>;
 
 /* ************************************************************************* */
 void HessianFactor::Allocate(const Scatter& scatter) {