New Residual Model and Fix relative import

* Adding Residual MLP
* Adding test Residual MLP
* Modified relative import Continuous Conv

tests/test_model/test_residualfnn.py

@@ -0,0 +1,22 @@
+import torch
+import pytest
+from pina.model import ResidualFeedForward
+
+def test_constructor():
+    # simple constructor
+    ResidualFeedForward(input_dimensions=2, output_dimensions=1)
+
+    # wrong transformer nets (not 2)
+    with pytest.raises(ValueError):
+        ResidualFeedForward(input_dimensions=2, output_dimensions=1, transformer_nets=[torch.nn.Linear(2, 20)])
+
+    # wrong transformer nets (not nn.Module)
+    with pytest.raises(ValueError):
+        ResidualFeedForward(input_dimensions=2, output_dimensions=1, transformer_nets=[2, 2])
+
+def test_forward():
+    x = torch.rand(10, 2)
+    model = ResidualFeedForward(input_dimensions=2, output_dimensions=1)
+    model(x)
+
+