Fix waveform generator caching issue

Small issue that occurs when you modify the time_domain_source_model. It still uses the cached frequency_domain_waveform instead of calculating it again. Conversely, if you swap the frequency_domain_source_model and calculate time domain strains, the same issue occurs. I have been using this because it was easier in my postprocessing script to only have to deal with one likelihood in which I swap the source models.