`inject_signal` arguments

@colm.talbot @moritz.huebner @paul-lasky instead of passing waveform_polarizations into inject_signal could we just pass in the waveform_generator? Am I missing something or is this forcing the user to have to call

hf_signal = waveform_generator.frequency_domain_strain()

everytime?

A related issue is that docstring for get_interferoemeters_with_fake_noise_and_injection() is out of date and references waveform_generator.get_frequency_domain_signal().