Fix logic with counter in Inversion (#56)

Fix bug introduced in the last commit. Now the counter is always set to
the last iteration that was run. Before the zeroth iteration, the
`Inversion` doesn't have any counter.

Author: santisoler

src/inversion_ideas/inversion.py

@@ -86,14 +86,15 @@ def __init__(
         # Assign model as a copy of the initial model
         self.model = initial_model.copy()
 
-        # Initialize the counter
-        self._counter = 0
-
     def __next__(self):
         """
         Run next iteration in the inversion.
         """
-        if self.counter == 0:
+        # Zeroth iteration
+        if not hasattr(self, "_counter"):
+            # Initialize counter to zero
+            self._counter = 0
+
             # Add initial model to log (only on zeroth iteration)
             if self.log is not None:
                 self.log.update(self.counter, self.model)
@@ -102,9 +103,6 @@ def __next__(self):
             if hasattr(self.stopping_criteria, "initialize"):
                 self.stopping_criteria.initialize()
 
-            # Increase counter by one
-            self._counter += 1
-
             # Return the initial model in the zeroth iteration
             return self.model