Adds new class GWSignalWaveformGenerator for interfacing GWSignal.
Allows choosing the generator from the dataset settings.
To be more tested: calling LAL models from the GWSignalWaveformGenerator, but this is not crucial for O4a since LAL models will still be callable through the LALSim interfaces.
This MR introduces:
To be more tested: calling LAL models from the GWSignalWaveformGenerator, but this is not crucial for O4a since LAL models will still be callable through the LALSim interfaces.