FDS Source: fix output format for near exact powers of 10

[rmcdermo]

This PR corrects output that looks like this:

 Time Step:     900, Simulation Time: 9.0000000 s
 Time Step:    1000, Simulation Time:10.0000000 s
 Time Step:    1100, Simulation Time: 11.000000 s

to look like this:

 Time Step:     900, Simulation Time: 9.0000000 s
 Time Step:    1000, Simulation Time: 10.000000 s
 Time Step:    1100, Simulation Time: 11.000000 s