rename diffraction_type to processing_type, use enumeration and add m…

…ore descriptions

contributed_definitions/NXstress.nxdl.xml

@@ -128,13 +128,41 @@
                 Brief summary of the collection, including grouping criteria. The information provided in this field can highlight, for example, the measurement setup or information about experimental conditions.
             </doc>       
         </field>
-        <field name="diffraction_type" type="NX_CHAR" optional="false">
-            <doc>   This variable describes the type of data plotted in the diffractogram and describes the type of calculation used in the EASI-STRESS software. Any of these values are valid:
-                     * ``two-theta``
-                     * ``energy``
-                     * ``d-spacing``
-                     * ``sin2psi``
+        <field name="processing_type" type="NX_CHAR" optional="false">
+            <doc>
+                Describes the way strain `\varepsilon ` can be calculated from the :ref:`center &lt;/NXstress/ENTRY/peaks/center-field&gt;`
+                peak parameter.
             </doc>
+            <enumeration>
+                <item value="two-theta">
+                    <doc>
+                        \ :math:`\varepsilon = \large \frac{sin(\mathrm{\theta}_{0})}{sin(\mathrm{\theta})}-1`
+                    </doc>
+                </item>
+                <item value="energy">
+                    <doc>
+                        \ :math:`\varepsilon = \large \frac{\mathrm{E}_{0}}{\mathrm{E}}-1`
+                    </doc>
+                </item>
+                <item value="d-spacing">
+                    <doc>
+                        \ :math:`\varepsilon = \large \frac{\mathrm{d}}{\mathrm{d}_{0}}-1`
+                    </doc>
+                </item>
+                <item value="time-of-flight">
+                    <doc>
+                        \ :math:`\varepsilon = \large \frac{\mathrm{TOF}}{\mathrm{TOF}_{0}}-1`
+                    </doc>
+                </item>
+                <item value="sin2psi">
+                    <doc>
+                        A description of the \ :math:`\mathrm{\sin}^{2}\psi` method can be found in the literature. Two examples are:
+                        `Fitzpatrick, M. E. et. al.; Determination of residual stresses by X-ray diffraction; National Physical Laboratory, Teddington, 2005`
+                        and
+                        `EN 15305:2009 standard: Non-destructive testing - Test Method for Residual Stress analysis by X-ray Diffraction`.
+                    </doc>
+                </item>
+            </enumeration>
         </field>         
         <field name="measurement_direction" type="NX_CHAR" optional="true">
             <doc>