2D: add loss that minimizes the area of the triangle in 3D

This is similar to the default loss in the Learner1D

It results in:

adaptive/learner/average_learner.py

@@ -120,3 +120,9 @@ class AverageLearner(BaseLearner):
         num_bins = int(max(5, sqrt(self.npoints)))
         vals = hv.Points(vals)
         return hv.operation.histogram(vals, num_bins=num_bins, dimension=1)
+
+    def _get_data(self):
+        return (self.data, self.npoints, self.sum_f, self.sum_f_sq)
+
+    def _set_data(self, data):
+        self.data, self.npoints, self.sum_f, self.sum_f_sq = data