add save to VTK (#1004)

# Description
Please include a summary of the changes.

Relates to <issue>

# 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

examples/examples/geometries/a01_horizontal_stratigraphic.py

@@ -47,3 +47,88 @@
 # Plot the result of the model in the x and y direction with data and without boundaries
 gpv.plot_2d(geo_data, direction=['x'], show_data=True, show_boundaries=False)
 gpv.plot_2d(geo_data, direction=['y'], show_data=True, show_boundaries=False)
+
+# %%
+# Export to VTK model
+p = gpv.plot_3d(geo_data, show_data=True, show_results=True, show_boundaries=True,image=True)
+p.surface_poly['rock1'].save('rock1.vtk') # Save the vtk file for formation 1
+p.surface_poly['rock2'].save('rock2.vtk') # Save the vtk file for formation 2
+p.orientations_mesh.save('orientations.vtk') # Save the vtk file for the orientations
+p.surface_points_mesh.save('surface_points.vtk') # Save the vtk file for the surface points
+box = p.regular_grid_actor.GetMapper().GetInput() # Get the vtk file for the regular grid
+box.save('box.vtk')
+
+# %%
+import pyvista as pv
+pv.read('rock1.vtk').plot(show_edges=False)
+pv.read('rock2.vtk').plot(show_edges=False)
+pv.read('orientations.vtk').plot(show_edges=False)
+pv.read('surface_points.vtk').plot(show_edges=False)
+pv.read('box.vtk').plot(show_edges=False)
+
+# %%
+# Export vertices of mesh
+from builtins import range
+import vtk
+import pandas as pd
+def generate_normals(polydata):
+    normal_generator = vtk.vtkPolyDataNormals()
+    normal_generator.SetInputData(polydata)
+    normal_generator.ComputePointNormalsOn()
+    normal_generator.ComputeCellNormalsOff()
+    normal_generator.Update()
+    return normal_generator.GetOutput()
+
+def get_vertices_and_normals(mesh):
+
+    surface_mesh = mesh.extract_surface()
+    polydata = surface_mesh
+
+    # Generate normals if not present
+    polydata_with_normals = generate_normals(polydata)
+
+    # Get points (vertices)
+    points = polydata_with_normals.GetPoints()
+    vertices = []
+    for i in range(points.GetNumberOfPoints()):
+        vertices.append(points.GetPoint(i))
+
+    # Get normals
+    normals_array = polydata_with_normals.GetPointData().GetNormals()
+    normals = []
+    for i in range(normals_array.GetNumberOfTuples()):
+        normals.append(normals_array.GetTuple(i))
+
+    return vertices, normals
+
+def save_to_excel(vertices, normals, vertices_file, normals_file):
+    # Create DataFrames
+    vertices_df = pd.DataFrame(vertices, columns=['X', 'Y', 'Z'])
+    normals_df = pd.DataFrame(normals, columns=['x', 'y', 'z'])
+
+    # Save to Excel files
+    vertices_df.to_excel(vertices_file, index=False)
+    normals_df.to_excel
+mesh = p.surface_poly['rock1']
+vertices, normals = get_vertices_and_normals(mesh)
+vertices_df = pd.DataFrame(vertices, columns=['X', 'Y', 'Z'])
+normals_df = pd.DataFrame(normals, columns=['x', 'y', 'z'])
+# Save to Excel filesthe
+vertices_file = "rock1_vertices.xlsx"
+normals_file = "rock1_norms.xlsx"
+save_to_excel(vertices, normals, vertices_file, normals_file)
+pd.read_excel(vertices_file)
+pd.read_excel(normals_file)
+
+# %%
+# Convert the DataFrame to an XYZ file
+def dataframe_to_xyz(df, filename):
+    with open(filename, 'w') as f:
+        for index, row in df.iterrows():
+            f.write(f"{row['X']} {row['Y']} {row['Z']}\n")
+
+# Specify the filename
+filename = "output.xyz"
+
+# Call the function to write the DataFrame to an XYZ file
+dataframe_to_xyz(vertices_df, filename)