correlated_fields.py: Disable via `0.` not `None`

src/library/correlated_fields.py

@@ -443,9 +443,9 @@ class CorrelatedFieldMaker:
             in this call should hold.
         fluctuations_{mean,stddev} : float
             Total spectral energy -> Amplitude of the fluctuations
-        flexibility_{mean,stddev} : float or None
+        flexibility_{mean,stddev} : float
             Amplitude of the non-power-law power spectrum component
-        asperity_{mean,stddev} : float or None
+        asperity_{mean,stddev} : float
             Roughness of the non-power-law power spectrum component
             Used to accommodate single frequency peaks
         loglogavgslope_{mean,stddev} : float
@@ -487,20 +487,20 @@ class CorrelatedFieldMaker:
         prefix = str(prefix)
         # assert isinstance(target_subdomain[space], (RGSpace, HPSpace, GLSpace)
 
-        ve = "{0}_mean and {0}_stddev must be strictly positive (or both None)"
+        ve = "{0}_mean and {0}_stddev must be strictly positive (or both zero to disable {0})"
         if fluctuations_mean > 0. and fluctuations_stddev > 0.:
             fluct = LognormalTransform(fluctuations_mean, fluctuations_stddev,
                                        self._prefix + prefix + 'fluctuations', N)
         else:
             raise ValueError(ve.format("fluctuations"))
-        if flexibility_mean is None and flexibility_stddev is None:
+        if flexibility_mean == 0. and flexibility_stddev == 0.:
             flex = None
         elif flexibility_mean > 0. and flexibility_stddev > 0.:
             flex = LognormalTransform(flexibility_mean, flexibility_stddev,
                                       self._prefix + prefix + 'flexibility', N)
         else:
             raise ValueError(ve.format("flexibility"))
-        if asperity_mean is None and asperity_stddev is None:
+        if asperity_mean == 0. and asperity_stddev == 0.:
             asp = None
         elif asperity_mean > 0. and asperity_stddev > 0.:
             asp = LognormalTransform(asperity_mean, asperity_stddev,