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

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

tests/test_label_tensor.py

@@ -27,6 +27,7 @@ def test_labels():
 def test_extract():
     label_to_extract = ['a', 'c']
     tensor = LabelTensor(data, labels)
+    print(tensor)
     new = tensor.extract(label_to_extract)
     assert new.labels == label_to_extract
     assert new.shape[1] == len(label_to_extract)
@@ -79,3 +80,11 @@ def test_merge():
 
     tensor_bc = tensor_b.append(tensor_c)
     assert torch.allclose(tensor_bc, tensor.extract(['b', 'c']))
+
+
+def test_getitem():
+    tensor = LabelTensor(data, labels)
+    tensor_view = tensor[:5]
+
+    assert tensor_view.labels == labels
+    assert torch.allclose(tensor_view, data[:5])