make waveform generator class saved in GWT meta data

This MR allows the CBCResult plot to work with non-standard waveform generator classes, as long as the new class takes the same initialisation arguments.

test/gw_likelihood_test.py

@@ -156,6 +156,7 @@ class TestGWTransient(unittest.TestCase):
             time_marginalization=False,
             phase_marginalization=False,
             distance_marginalization=False,
+            waveform_generator_class=self.waveform_generator.__class__,
             waveform_arguments=self.waveform_generator.waveform_arguments,
             frequency_domain_source_model=self.waveform_generator.frequency_domain_source_model,
             parameter_conversion=self.waveform_generator.parameter_conversion,