Fix Codacy Warnings (#477)

---------

Co-authored-by: Dario Coscia <dariocos99@gmail.com>

pina/optim/optimizer_interface.py

@@ -1,15 +1,24 @@
-"""Module for PINA Optimizer"""
+"""Module for PINA Optimizer."""
 
 from abc import ABCMeta, abstractmethod
 
 
-class Optimizer(metaclass=ABCMeta):  # TODO improve interface
+class Optimizer(metaclass=ABCMeta):
+    """
+    TODO
+    :param metaclass: _description_, defaults to ABCMeta
+    :type metaclass: _type_, optional
+    """
 
     @property
     @abstractmethod
     def instance(self):
-        pass
+        """
+        TODO
+        """
 
     @abstractmethod
     def hook(self):
-        pass
+        """
+        TODO
+        """