[CLN] Rename regular_grid to _regular_grid in Topography

Refactored `regular_grid` to `_regular_grid` to indicate it as a private attribute. This change ensures better encapsulation and aligns with naming conventions.

Author: Leguark

gempy/API/grid_API.py

@@ -59,7 +59,7 @@ def set_topography_from_random(grid: Grid, fractal_dimension: float = 2.0, d_z:
     )
 
     grid.topography = Topography(
-        regular_grid=grid.regular_grid,
+        _regular_grid=grid.regular_grid,
         values_2d=random_topography
     )
 

gempy/core/data/grid_modules/topography.py

@@ -23,7 +23,7 @@ class Topography:
           This always assumes that the topography we pass fits perfectly the extent.
       """
 
-    regular_grid: RegularGrid
+    _regular_grid: RegularGrid
     values_2d: np.ndarray = Field(exclude=True, default_factory=lambda: np.zeros((0, 0, 3)))
     source: Optional[str] = None