Expose `survive_install_run_time_fn`

[Tuntenfisch]

Hey,

I'm writing a ROS2 node that integrates libsurvive and publishes the tracked poses inside ROS2. In ROS2 you can use the parameter use_sim_time to provide a time basis different from the wall clock time.

I noticed that libsurvive already has the function survive_install_run_time_fn internally to change its time basis. Exposing this function to the public API would allow users of the library to also change the time basis.

[bl4ckb0ne]

I don't see anything against that. Moving it to include/libsurvive/survive.h would expose it to the public api. Could you send a pull request please?