CMake option to build partition/metis

[Andrew Melim]

@fdellaert Do we want to insert a cmake flag for compiling metis/partition in gtsam_unstable? All the dependencies are included in 3rd party so the only downside is a minor increase in compilation time.

[Frank Dellaert]

Is metis not in third party?

[Andrew Melim]

Yes, it's in third party. I was just unsure on the standard practice re: building it if it's only used in gtsam_unstable.