[CLN] Refactor grid selection logic by using active state checks

Updated the grid selection logic to use `dense_grid_exists_and_active` and `octree_grid_exists_and_active` checks instead of direct `None` comparisons for consistency and readability.

Author: Leguark

gempy/core/data/grid.py

@@ -275,9 +275,9 @@ def regular_grid(self):
 
         if dense_grid_exists_and_active and octree_grid_exists_and_active:
             raise AttributeError('Both dense_grid and octree_grid are active. This is not possible.')
-        elif self.dense_grid is not None:
+        elif dense_grid_exists_and_active:
             return self.dense_grid
-        elif self.octree_grid is not None:
+        elif octree_grid_exists_and_active:
             return self.octree_grid
         else:
             return None