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

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

tests/test_cartesian.py

@@ -0,0 +1,21 @@
+import torch
+import pytest
+
+from pina import LabelTensor, Condition, CartesianDomain, PINN
+from pina.problem import SpatialProblem
+from pina.model import FeedForward
+from pina.operators import nabla
+
+
+
+def test_constructor():
+    CartesianDomain({'x': [0, 1], 'y': [0, 1]})
+
+
+def test_is_inside():
+    pt_1 = LabelTensor(torch.tensor([[0.5, 0.5]]), ['x', 'y'])
+    pt_2 = LabelTensor(torch.tensor([[1.0, 0.5]]), ['x', 'y'])
+    pt_3 = LabelTensor(torch.tensor([[1.5, 0.5]]), ['x', 'y'])
+    domain = CartesianDomain({'x': [0, 1], 'y': [0, 1]})
+    for pt, exp_result in zip([pt_1, pt_2, pt_3], [True, True, False]):
+        assert domain.is_inside(pt) == exp_result