Convert the frequency_domain_source model to a string in the CBCResult

Previously, the function was added to the save data dictionary.
Eventually causing the function to be pickled and all CBCResults to be
dependent on bilby. This converts it to a string so that people at least
know which source model was used.

bilby/gw/likelihood.py

@@ -73,7 +73,7 @@ class GravitationalWaveTransient(likelihood.Likelihood):
             phase_marginalization=self.phase_marginalization,
             distance_marginalization=self.distance_marginalization,
             waveform_arguments=waveform_generator.waveform_arguments,
-            frequency_domain_source_model=waveform_generator.frequency_domain_source_model)
+            frequency_domain_source_model=str(waveform_generator.frequency_domain_source_model))
 
         if self.time_marginalization:
             self._check_prior_is_set(key='geocent_time')