[ENH] Add grouping option to structural elements initialization

Introduce a `group_by` parameter with `Literal` typing in `structural_elements_from_borehole_set` for flexible grouping. Also, add dotenv initialization in the example script to manage environment variables.

Author: Leguark

examples/examples/_aux_func_petrophysics_inversion.py

@@ -12,9 +12,11 @@
 def generate_spremberg_model(
         borehole_set: ss.core.geological_formats.BoreholeSet,
         elements_to_gempy: dict[str, dict[str, str]], plot: bool = False) -> gp.data.GeoModel:
+    
     elements: list[gp.data.StructuralElement] = gp.structural_elements_from_borehole_set(
         borehole_set=borehole_set,
-        elements_dict=elements_to_gempy
+        elements_dict=elements_to_gempy,
+        group_by="component lith"
     )
 
     group = gp.data.StructuralGroup(

examples/examples/petrophysics_inversion.py

@@ -6,12 +6,16 @@
 
 # sphinx_gallery_thumbnail_number = -1
 """
+import dotenv
 import numpy as np
 
 import gempy as gp
 import gempy_viewer as gpv
 
 from examples.examples._aux_func_petrophysics_inversion import get_spremberg_borehole_set, generate_spremberg_model
+from subsurface.modules.visualization import to_pyvista_line, to_pyvista_points
+
+dotenv.load_dotenv()
 
 # %%
 vertical_exaggeration = 3