Made "model" a field

For the following reasons:
i) model is also a parameter, cuz it can be used to instruct programs to follow specific execution branches
ii) currently the NeXus documentation building as issues with dealing with groups where an attribute and field with the same concept name appear, see conversation of the #1560 for more details about this, what I think is a bug.

Author: mkuehbach

base_classes/NXparameters.nxdl.xml

@@ -41,7 +41,7 @@
         have been defined to store metadata associated with such a
         refinement.       
     </doc>
-    <attribute name="model">
+    <field name="model">
         <doc>
             The name of the model used in optimizing the parameter
             values. Fitting packages such as LMFIT
@@ -50,7 +50,7 @@
             attribute is provided, it is assumed that all the parameters
             in this group are associated with this model.
         </doc>
-    </attribute>
+    </field>
     <field name="PARAMETER" type="NX_CHAR_OR_NUMBER" nameType="any">
         <doc>
             A parameter that is used in or results from processing.