Set `start_time` more sensibly for injections

This code snippet:

ifos.set_strain_data_from_power_spectral_densities(
            sampling_frequency=self.sampling_frequency,
            duration=self.duration,
            **start_time=self.trigger_time - self.duration / 2**,
        )

This means that the start_time for injections is not consistent with the start_time for events.