[CLN] Refactor grid selection logic by using active state checks (#1062)

# Description
Refactored the `regular_grid` method to use the previously defined boolean variables `dense_grid_exists_and_active` and `octree_grid_exists_and_active` for consistency and improved readability, instead of directly checking if the grid objects are not None.

Relates to #grid-refactoring

# Checklist
- [ ] My code uses type hinting for function and method arguments and return values.
- [ ] I have created tests which cover my code.
- [ ] The test code either 1. demonstrates at least one valuable use case (e.g. integration tests) 
or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
- [ ] New tests pass locally with my changes.

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