Adding 3D fixed-source urban source example

.github/workflows/nightly.yaml

@@ -27,4 +27,5 @@ jobs:
         shell: bash
         run: |
           module load opensn/clang/17 python3/3.12.3
-          cd OpenSn_Logo_CAD && mpirun -np 8 ../opensn/build/python/opensn -i opensn.py
+          cd OpenSn_Logo_CAD && mpirun -np 8 ../opensn/build/python/opensn -i opensn.py && cd ..
+          cd Urban_Source && mpirun -np 16 ../opensn/build/python/opensn -i urban_source.py

README.md

@@ -7,3 +7,5 @@ A short description of each applicaiton/example is given below:
 
 - **OpenSn_Logo_CAD**  
     3D fixed-source problem with a source region in the shape of the OpenSn logo
+- **Urban_Source**  
+    3D fixed-source problem that models a source in an urban setting

Urban_Source/README.md

@@ -0,0 +1,27 @@
+## OpenSn Urban Source Example
+
+This example is a 3D, fixed-source problem with one energy group. The problem models an urban setting
+with a random number of concrete buldings of various size and shape. A single source is placed in a 
+random location outside a building. The entire model is enclosed in a block of air. The OpenSCAD
+script will generate a new random building/source layout every time it is executed.
+
+The example illustrates a complete CAD-to-OpenSn workflow:
+
+1. The base model is created in OpenSCAD ([urban_source.scad](./urban_source.scad))
+2. OpenSCAD exports the model as a CSG file ([urban_source.csg](./urban_source.csg))
+3. The CSG file is imported into FreeCAD and exported to a STEP File (urban_source.step)
+4. The STEP file is meshed using Gmsh ([urban_source.geo](./urban_source.geo)) and the resulting
+   `.msh` file can be read directly by OpenSn
+
+The Python OpenSn input file for this example can be found here: [opensn.py](./opensn.py).
+
+<figure>
+  <img src="urban_source_cad.png" alt="CAD Model" width="600"/>
+  <figcaption>CAD Model</figcaption>
+  <br>
+  <img src="urban_source_grid.png" alt="OpenSn with grid" width="600"/>
+  <figcaption>OpenSn Results with Grid</figcaption>
+  <br>
+  <img src="urban_source_nogrid.png" alt="OpenSn without grid" width="600"/>
+  <figcaption>OpenSn Results without Grid</figcaption>
+</figure>