Cast Eigen::Matrix dimension type to int

[varunagrawal]

When compiling GTSAM with clang (particularly for GPMP2) and -Wc++11-narrowing enabled (which clang is more strict about), I am getting the error

/opt/homebrew/lib/cmake/GTSAM/../../../include/gtsam/base/ProductLieGroup.h:100:33: error: non-type template argument evaluates to 18446744073709551615, which cannot be narrowed to type 'int' [-Wc++11-narrowing]
  typedef Eigen::Matrix<double, dimension2, dimension2> Jacobian2;
                                ^

since dimension2 is of type size_t and the template type for Eigen::Matrix is int.

ChatGPT confirmed this is a signed vs unsigned int issue and I applied a static_cast as it recommended.

Also replaced typedef with using for Jacobian type aliases.

[dellaert]

LGTM, thanks!