add how to use to the documentation

docs/example_of_use.rst

@@ -0,0 +1,107 @@
+Example of use
+==============
+
+The following is an example of how to use GEOUNED to convert a CAD file to a CSG file 
+for use in MCNP. It includes the methodology and rationale behind the process.
+
+.. warning:: It is assumed that GEOUNED has been properly installed as described in the 
+    `Quick install guide <./quick_install_guide.html>`_.
+
+1. `CAD simplification`_
+2. `Export as STEP`_
+3. `Conversion to CSG`_
+4. `Suspicious solids and debug`_
+5. `Void check verification`_
+
+.. _CAD simplification:
+
+CAD simplification
+==================
+
+The CAD model of the system should undergo a process of simplification. In this process,
+features that do not affect the radiation transport should be removed. The level of 
+detail is usually decreased. Surfaces that are not allowed like splines are substituted
+by simpler surfaces like planes or cylinders.
+
+.. _Export as STEP:
+
+Export as STEP
+==============
+
+Once the CAD is simplified, it is exported as a STEP file to be used as input by 
+GEOUNED.
+
+.. _Conversion to CSG:
+
+Conversion to CSG
+=================
+
+The below code is run in a Python environment with GEOUNED installed. In the comments
+of the code there is an explanation of the parameters used.
+
+.. code-block:: python
+
+    import geouned
+
+    settings = geouned.Settings(
+        debug=True,  # This will create a folder with the CAD bodies and their decomposition
+        startCell=12001,  # The first cell id
+        startSurf=12001,  # The first surface id
+        compSolids=False,  # False so the different solids of a component 
+                           # are not combined into a single MCNP cell
+        voidGen=True,  # Generate the void cells 
+    )
+
+    geo = geouned.CadToCsg(settings=settings)
+    geo.load_step_file(filename=f"Geometry/model.stp")  # Path to the STEP file
+    geo.start()
+    print("Start exporting")
+    geo.export_csg(
+        geometryName="My model",  # Name of the model to appear in the MCNP file
+        outFormat=("mcnp",),  # The output format
+        UCARD=50,  # Makes the model a filler universe with filler id 50
+        volCARD=True,  # The volume card is included in the cell definition
+        volSDEF=True,  # Generate a SDEF card
+    )
+    print("Exporting finished")
+
+.. _Suspicious solids and debug:
+
+Suspicious solids and debug
+===========================
+
+Running the above code will generate a debug folder that contains each of the converted 
+solids, both the original and the decomposed ones. Those bodies can be opened and 
+examined in a CAD software to compare them to the original model. If the decomposed 
+model doesn't look right, it is possible to manually decompose a specific solid and 
+redo the conversion.
+
+Also, it is possible that a folder called *Suspicious solids* is created. This folder 
+contains the same solids as the debug folder but only the ones that have been flagged as
+potentially problematic. This can be useful to quickly identify the problematic solids.
+
+After applying any fix necessary and exporting to STEP, the conversion can be run again.
+
+.. _Void check verification:
+
+Void check verification
+=======================
+
+Once the conversion is finished, it is recommended to stocastically check the volume
+of the solids and to make some plots.
+
+In this example, the *UCARD* was used to make the model a filler universe. For these 
+initial checks the *U=50* cards should be commented out as we want to test the 
+standalone model. Plots with the MCNP plotter should be done to check that the model 
+looks as expected. 
+
+Also, a void check simulation should be run. The *volSDEF=True* card automatically 
+generated a spherical inward-oriented source with the correct weight to calculate 
+volumes stochastically. The tallies for the solid MCNP cells were also automatically 
+generated. The user can run the simulation directly and the results of the tallies 
+should all be close to 1.0. If a MCNP cell tally shows a result far from 1.0 it would 
+mean that the volume of the solid is not correct.
+
+.. warning:: The void check simulation should be run with a high number of particles to 
+    achieve an acceptable statistical error. An usually number is 1e9 particles. Always
+    check that the statistical error of the tallies is below 0.1.

docs/index.rst

@@ -11,6 +11,7 @@ Supported codes include OpenMC, PHITS, Serpent and MCNP.
    :maxdepth: 2
 
    quick_install_guide
+   example_of_use
    users_guide/index
    python_api
    developers_guide