Added a readme example that was also made into a test, and restructured a bit

Author: jmwright

README.md

@@ -26,14 +26,40 @@ import cadquery_assembly_mesh_plugin.plugin
 You can then tag faces in each of the assembly parts and create your asembly. To export the assembly to a mesh file, you do the following.
 
 ```python
-your_assembly.assemblyToGmsh(mesh_path="tagged_planar_coil.msh")
+your_assembly.assemblyToGmsh(mesh_path="tagged_mesh.msh")
 ```
 
 Normal tag names lead to a physical group with the assembly part name prefixed. So a tag name of `inner-bottom` on an assembly part with the name `steel_plate` will be `steel_plate_inner-bottom`
 
 By prefixing a tag with the `~` character, the part name is ignored, which allows for tagging of a multi-material
 physical group. For instance, tagging multiple faces with `~contact-with-casing` will produce a physical group with the name `contact-with-casing` that includes all those faces, even if they belong to different parts/solids.
 
+Below is a simple example.
+
+```python
+import cadquery as cq
+import cadquery_assembly_mesh_plugin.plugin
+import gmsh
+
+shell = cq.Workplane("XY").box(50, 50, 50)
+shell = shell.faces(">Z").workplane().rect(21, 21).cutThruAll()
+shell.faces(">X[-2]").tag("inner-right")
+shell.faces("<X[-2]").tag("~in_contact")
+
+# Create the insert
+insert = cq.Workplane("XY").box(20, 20, 50)
+insert.faces("<X").tag("~in_contact")
+insert.faces(">X").tag("outer-right")
+
+assy = cq.Assembly()
+assy.add(shell, name="shell", loc=cq.Location(cq.Vector(0, 0, 0)), color=cq.Color("red"))
+assy.add(insert, name="insert", loc=cq.Location(cq.Vector(0, 0, 0)), color=cq.Color("blue"))
+
+assy.assemblyToGmsh(mesh_path="tagged_mesh.msh")
+```
+
+The resulting `.msh` file should have three physical groups in it. The `in_contact` group should include the faces from both the shell and the insert.
+
 ## Tests
 
 These tests are also run in Github Actions, and the meshes which are generated can be viewed as artifacts on the successful `tests` Actions there.

tests/sample_coils.py renamed to tests/sample_assemblies.py

@@ -1,6 +1,36 @@
 import cadquery as cq
 
 
+def generate_simple_nested_boxes():
+    """
+    Generates the simplest assembly case where two boxes are nested inside each other.
+    """
+
+    # Create the outter shell
+    shell = cq.Workplane("XY").box(50, 50, 50)
+    shell = shell.faces(">Z").workplane().rect(21, 21).cutThruAll()
+    shell.faces(">X[-2]").tag("inner-right")
+    shell.faces("<X[-2]").tag("~in_contact")
+
+    # Create the insert
+    insert = cq.Workplane("XY").box(20, 20, 50)
+    insert.faces("<X").tag("~in_contact")
+    insert.faces(">X").tag("outer-right")
+
+    assy = cq.Assembly()
+    assy.add(
+        shell, name="shell", loc=cq.Location(cq.Vector(0, 0, 0)), color=cq.Color("red")
+    )
+    assy.add(
+        insert,
+        name="insert",
+        loc=cq.Location(cq.Vector(0, 0, 0)),
+        color=cq.Color("blue"),
+    )
+
+    return assy
+
+
 def generate_test_cross_section():
     """
     Generates a basic cross-section to verify that the tagged faces are crossing over

tests/smoke_test.py

@@ -1,5 +1,21 @@
 import assembly_mesh_plugin.plugin
-from sample_coils import generate_test_cross_section, generate_assembly
+from tests.sample_assemblies import (
+    generate_simple_nested_boxes,
+    generate_test_cross_section,
+    generate_assembly,
+)
+
+
+def test_basic_assembly():
+    """
+    Tests to make sure that the most basic assembly works correctly with tagging.
+    """
+
+    # Create the basic assembly
+    assy = generate_simple_nested_boxes()
+
+    # Create a mesh that has all the faces tagged as physical groups
+    assy.assemblyToGmsh(mesh_path="tagged_mesh.msh")
 
 
 def test_basic_cross_section():