Generalise the WaveformGenerator to accept a `source_model` as a class method

Original title: Parameter inference for source models fails for methods

The WaveformGenerator class attempts to infer the parameters of a source model using the utils.infer_parameters_from_function function, however, if this class is passed a method it will fail, thanks to the self argument of the method. I notice there's a utils.infer_args_from_method as well; could the correct one be determined automatically by the WaveformGenerator?