fix for interface change in galsim v2.0.7

Author: jchiang87

python/desc/imsim/atmPSF.py

@@ -33,7 +33,7 @@ def _wavefront_gradient(self, u, v, t, theta):
         Z = galsim.OpticalScreen(diam=8.36, obscuration=0.61, aberrations=[0]*4+list(z))
         return Z._wavefront_gradient(u, v, t, theta)
 
-    def _stepK(self, **kwargs):
+    def _getStepK(self, **kwargs):
         return self.stepk