JumpTime, Iterations/frame, units

Finalized units--including time which is now displayed in days.
Changed jumpTime method to be more accurate with orbitals
Changed fast forward to iterations/frame
Encapsulated gravity calculations into one method
You usually only want to do NULL checks on pointers. In C/C++ you can generally assume NULL == 0. Therefore your function would fail if they were trying pass in inputFloat with a value 0 into a string.