use NXparameters where appropriate

contributed_definitions/NXstress.nxdl.xml

@@ -456,7 +456,7 @@
         	        </enumeration>
                 </attribute>
 	        </field>
-            <group name="gauge_volume" type="NXcollection" optional="true">
+            <group name="gauge_volume" type="NXparameters" optional="true">
                 <doc> The gauge volume can be described with the following parameters:
                  .. figure:: stress/GV2.jpg
                   :width: 70%
@@ -563,13 +563,10 @@
                     The name will be numbered to allow for ordering of steps.
                  </doc>
             </group>
-            <group name="peak_parameters" type="NXdata" optional="false">
+            <group name="peak_parameters" type="NXparameters" optional="false">
                 <doc>
                     This group contains all diffraction peak fit parameters. 
                     This information is not required for stress and strain calculations.
-                    Note that as in any *NXdata* group, each field can have uncertainties
-                    associated to them (e.g. *center_errors* would be the uncertainties
-                    on the peak center).
                 </doc>
                 <field name="title" type="NX_CHAR" optional="false">
                     <doc>Diffraction peak profile.</doc>
@@ -721,10 +718,10 @@
                     </dimensions>
                 </field>
             </group>
-            <group name="background_parameters" type="NXdata" optional="false">
+            <group name="background_parameters" type="NXparameters" optional="false">
                 <doc>
                     This group contains all background fit parameters. 
-                    This information is not required for stress and strain caluclations.
+                    This information is not required for stress and strain calculations.
                 </doc>
                 <field name="title" type="NX_CHAR" optional="false">
                     <doc>