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.

bilby/gw/result.py

@@ -11,7 +11,6 @@ import numpy as np
 from ..core.result import Result as CoreResult
 from ..core.utils import infft, logger, check_directory_exists_and_if_not_mkdir
 from .utils import plot_spline_pos, spline_angle_xform, asd_from_freq_series
-from .waveform_generator import WaveformGenerator
 from .detector import get_empty_interferometer, Interferometer