Solving problems related to Geometry (#118)

* fix and add tests
* minor fix on domain classes

---------

Co-authored-by: Dario Coscia <dariocoscia@cli-10-110-0-208.WIFIeduroamSTUD.units.it>
Co-authored-by: Dario Coscia <dariocoscia@dhcp-040.eduroam.sissa.it>

pina/geometry/difference_domain.py

@@ -16,13 +16,13 @@ class Difference(Location):
     def sample(self, n, mode='random', variables='all'):
         """
         """
-        assert mode is 'random', 'Only random mode is implemented'
+        assert mode == 'random', 'Only random mode is implemented'
 
         samples = []
         while len(samples) < n:
             sample = self.first.sample(1, 'random')
             if not self.second.is_inside(sample):
-                samples.append(sample.tolist()[0])
+                samples.append(sample)
 
         import torch
-        return LabelTensor(torch.tensor(samples), labels=['x', 'y'])
+        return LabelTensor(torch.cat(samples), labels=['x', 'y'])