MIONet addition and DeepONet modification.

* Create MIONet base class
* DeepONet inherits from MIONet

pina/model/__init__.py

@@ -2,10 +2,11 @@ __all__ = [
     'FeedForward',
     'MultiFeedForward',
     'DeepONet',
+    'MIONet',
     'FNO',
 ]
 
 from .feed_forward import FeedForward
 from .multi_feed_forward import MultiFeedForward
-from .deeponet import DeepONet
+from .deeponet import DeepONet, MIONet
 from .fno import FNO