New Residual Model and Fix relative import

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

pina/model/__init__.py

@@ -1,12 +1,13 @@
 __all__ = [
     'FeedForward',
+    'ResidualFeedForward',
     'MultiFeedForward',
     'DeepONet',
     'MIONet',
     'FNO',
 ]
 
-from .feed_forward import FeedForward
+from .feed_forward import FeedForward, ResidualFeedForward
 from .multi_feed_forward import MultiFeedForward
 from .deeponet import DeepONet, MIONet
 from .fno import FNO