equation class, fix minor bugs, diff domain (#89)

* equation class
* difference domain
* dummy dataloader
* writer class
* refactoring and minor fix

pina/geometry/difference_domain.py

@@ -0,0 +1,27 @@
+"""Module for Location class."""
+
+from .location import Location
+from ..label_tensor import LabelTensor
+
+class Difference(Location):
+    """
+    """
+    def __init__(self, first, second):
+
+
+        self.first = first
+        self.second = second
+
+    def sample(self, n, mode ='random', variables='all'):
+        """
+        """
+        assert mode is '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])
+
+        import torch
+        return LabelTensor(torch.tensor(samples), labels=['x', 'y'])