BUGFIX: Fix whitening procedure

Currently the attribute interferometer.whitened_frequency_domain_strain does not return strain with unit variance. This MR fixes this attribute and the ensuing plotting methods of the CBCResult object where it was called so that those plot the whitened time-domain strain with unit variance. Should I add a unit test that the interferometer.whitened_frequency_domain_strain actually returns unit-variance strain?

bilby/gw/detector/interferometer.py

@@ -618,13 +618,17 @@ class Interferometer(object):
 
     @property
     def whitened_frequency_domain_strain(self):
-        """ Calculates the whitened data by dividing data by the amplitude spectral density
+        """ Calculates the whitened data by dividing the frequency domain data by
+        ((amplitude spectral density) * (duration / 4) ** 0.5). The resulting
+        data will have unit variance.
 
         Returns
         =======
         array_like: The whitened data
         """
-        return self.strain_data.frequency_domain_strain / self.amplitude_spectral_density_array
+        return self.strain_data.frequency_domain_strain / (
+            self.amplitude_spectral_density_array * np.sqrt(self.duration / 4)
+        )
 
     def save_data(self, outdir, label=None):
         """ Creates save files for interferometer data in plain text format.