Use a static variable to count instead of machine time for simulation time intervals when using VELOCIraptor

Better naming convention for VELOCIraptor-related i/o parameters. Use the correct value for the time of first output in engine_compute_next_stf_time()

Put the checks for i/o dumps into a separate function. Call that function after the 0th step as well.

Make the STF dump follow the same pattern as the other dumps. A drift_all will have to be added one day.