[BUG] Outliers in gempy v3 (#1009)

# Description
Since the release of gempy v3 we noticed some outliers in the lithology block. These are basically single cells that have a wrong element ID in the lithology block. They generally appear close to surface boundaries and are appearing depending on resolution. This might be caused when mapping sclar field values to int for the lithology block.
As I don't think the isse is related to Octrees or DC I opened a new pull request. This issue will cause problems when reintroducing MC as a meshing approach in #1000 and #1006 .

For now I added a single example as a test (see screenshot). I will add more examples whe I come across them and have time.

![image](https://github.com/user-attachments/assets/5703b874-c566-4f7a-9771-bdaa2db7e2de)

# Checklist
- [ ] My code uses type hinting for function and method arguments and return values.
- [x] I have created tests which cover my code.
- [x] 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

test/test_QA/__init__.py

@@ -0,0 +1,78 @@
+import gempy as gp
+from gempy.optional_dependencies import require_gempy_viewer
+from gempy.core.data.enumerators import ExampleModel
+from gempy.core.data.grid_modules import RegularGrid
+import numpy as np
+
+PLOT = True
+
+
+def test_outliers_model_1():
+    # Path to input data
+    data_path = "https://raw.githubusercontent.com/cgre-aachen/gempy_data/master/"
+    path_to_data = data_path + "/data/input_data/video_tutorials_v3/"
+
+    # Create instance of geomodel
+    model = gp.create_geomodel(
+        project_name='tutorial_model_onlap_1',
+        extent=[0, 2000, 0, 1000, 0, 1000],
+        resolution=[100, 50, 50],
+        importer_helper=gp.data.ImporterHelper(
+            path_to_orientations=path_to_data + "tutorial_model_onlap_1_orientations.csv?cache=",
+            path_to_surface_points=path_to_data + "tutorial_model_onlap_1_surface_points.csv?cache="
+        )
+    )
+
+    # Map geological series to surfaces
+    gp.map_stack_to_surfaces(
+        gempy_model=model,
+        mapping_object={
+                "Young_Series": ("basin_fill_2", "basin_fill_1"),
+                "Old_Series"  : ("basin_top", "basin_bottom")
+        }
+    )
+
+    # Set the relation of the youngest group to Onlap
+    from gempy_engine.core.data.stack_relation_type import StackRelationType
+    model.structural_frame.structural_groups[0].structural_relation = StackRelationType.ONLAP
+    
+    model.interpolation_options.sigmoid_slope = 5_000_000
+
+    # Compute a solution for the model
+    gp.compute_model(
+        gempy_model=model,
+        engine_config=gp.data.GemPyEngineConfig(
+            backend=gp.data.AvailableBackends.numpy
+        )
+    )
+
+    # Assert
+    arrays = model.solutions.raw_arrays  # * arrays is equivalent to gempy v2 solutions
+    assert arrays.scalar_field_matrix.shape == (2, 250_000)  # * 2 groups, 250000 points
+
+    if PLOT:
+        gpv = require_gempy_viewer()
+        gpv.plot_2d(
+            model=model,
+            show_data=True,
+            show_boundaries=False,
+            show=True
+        )
+
+        gpv.plot_2d(
+            model=model,
+            show_data=False,
+            show_boundaries=False,
+            show_scalar=True,
+            show=True
+        )
+
+
+        gpv.plot_2d(
+            model=model,
+            show_data=False,
+            show_boundaries=False,
+            show_scalar=True,
+            series_n=1,
+            show=True
+        )