Resolve "(LearnerND) add iso-surface plot feature"

Closes #112 (closed)

adaptive/learner/learnerND.py

@@ -577,11 +577,6 @@ class LearnerND(BaseLearner):
         else:
             raise ValueError("Only 1 or 2-dimensional plots can be generated.")
 
-    def get_range(self):
-        r_min = min(self.data[v] for v in self.tri.vertices)
-        r_max = max(self.data[v] for v in self.tri.vertices)
-        return r_min, r_max
-
     def _get_isosurface(self, level=0.0):
         if self.ndim != 3 or self.vdim != 1:
             raise Exception('Isosurface plotting is only supported'
@@ -633,7 +628,8 @@ class LearnerND(BaseLearner):
                 faces.append(plane[1:])
 
         if len(faces) == 0:
-            r_min, r_max = self.get_range()
+            r_min = min(self.data[v] for v in self.tri.vertices)
+            r_max = max(self.data[v] for v in self.tri.vertices)
 
             raise ValueError(
                 f"Could not draw isosurface for level={level}, as"