Injecting into Gaussian noise at specific GPS times not working

From @ethan.payne, mentioned in #160 (closed) (modified slightly):

...after the injection file is generated with geocent_time's, bilby_pipe data generation and analysis generates the data at geocent_time prior with trigger_time = 0.

20:00 bilby_pipe INFO    : Injecting waveform with 
20:00 bilby_pipe INFO    : minimum_frequency = 20.0
20:00 bilby_pipe INFO    : maximum_frequency = 1024.0
20:00 bilby_pipe INFO    : trigger_time = 0.0
20:00 bilby_pipe INFO    : start_time = -2.0
20:00 bilby_pipe INFO    : duration = 4.0
...
20:00 bilby INFO    :   geocent_time = 1134423508.3953884