Update space_iterate.c

Acceleration is now as follows:
From impulse 0 to 50%, 10% impulse per tick. (5 seconds)
From 50-75%, 5% per tick. (5 seconds)
From 75-90%, 2.5% per tick (6 seconds)
From 90%-warp 1, 1% per tick (10 seconds.
Total time from 0 to warp is 26 seconds.

Warp acceleration is 1.0 factor up until cruise speed, 0/10th of that above cruise.

I also increased base signature for planets and stars.