Store the model as a class property of WaveformGenerator

When evaluating SEOBNRv5PHM through gwsignal, I think it would be useful to have access to the model, and hence the dynamics. I think this would involve passing debug=True when calling either generate_prec_hpc_opt or generate_modes_opt (in the WaveformGenerator.generate_td_polarizations or WaveformGenerator.generate_td_modes functions) and storing the model as self._model or alike. I am happy to put together an MR.