Add label to output file name with the frequency domain strain for each IFO.

As the save_to_file method of the interferometer is currently set up, the frequency domain data isn't saved using the run label, but rather just with the run output directory and the H1_frequency_domain_data.dat filename. This doesn't work for multiple runs with different labels getting saved to the same output directory. I just added a minor change so that the frequency domain data is saved with the same name format as the PSD data in the same function. (Sorry I didn't make this change on a dedicated feature branch.)