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

Closes #112 (closed)

adaptive/learner/learnerND.py

@@ -659,7 +659,7 @@ class LearnerND(BaseLearner):
         self.tell_many(*zip(*data.items()))
 
     def _get_iso(self, level=0.0, which='surface'):
-        if which == 'surface'
+        if which == 'surface':
             if self.ndim != 3 or self.vdim != 1:
                 raise Exception('Isosurface plotting is only supported'
                                 ' for a 3D input and 1D output')
@@ -671,8 +671,6 @@ class LearnerND(BaseLearner):
                                 ' for a 2D input and 1D output')
             get_surface = False
             get_line = True
-        else:
-            raise Exception
 
         vertices = []  # index -> (x,y,z)
         faces_or_lines = []  # tuple of indices of the corner points