Normalizer: implement simple derivative to remove scipy dependency

src/gstools/normalizer/base.py

@@ -13,10 +13,14 @@
 import warnings
 
 import numpy as np
-import scipy.misc as spm
 import scipy.optimize as spo
 
 
+def _derivative(f, x, dx=1e-6):
+    """Central difference formula."""
+    return (f(x + dx) - f(x - dx)) / (2 * dx)
+
+
 class Normalizer:
     """Normalizer class.
 
@@ -57,7 +61,7 @@ def _normalize(self, data):
         return data
 
     def _derivative(self, data):
-        return spm.derivative(self._normalize, data, dx=self._dx)
+        return _derivative(self._normalize, data, dx=self._dx)
 
     def _loglikelihood(self, data):
         add = -0.5 * np.size(data) * (np.log(2 * np.pi) + 1)