Fixed NXdirectof to have either a fermi_chopper or a disk_chopper

Renamed scaling attribute to scaling_factor in NXlog
Document readback and set value for fields in the manual, datarules.rst
Refs #375, #502, #315

applications/NXdirecttof.nxdl.xml

@@ -39,14 +39,26 @@
         </enumeration>
       </field>
      <group type="NXinstrument">
-       <group type="NXfermi_chopper" name="fermi_chopper">
+       <group type="NXfermi_chopper" name="fermi_chopper" minOccurs="0">
         <field name="rotation_speed" type="NX_FLOAT" units="NX_FREQUENCY">
           <doc>chopper rotation speed</doc>
         </field>
        <field name="energy" type="NX_FLOAT" units="NX_ENERGY">
             <doc>energy selected</doc>
        </field> 
        </group>
+       <group type="NXdisk_chopper" name="disk_chopper" minOccurs="0">
+        <field name="rotation_speed" type="NX_FLOAT" units="NX_FREQUENCY">
+          <doc>chopper rotation speed</doc>
+        </field>
+       <field name="energy" type="NX_FLOAT" units="NX_ENERGY">
+            <doc>energy selected</doc>
+       </field> 
+       </group>
+       <doc>
+          We definitly want the rotation_speed and energy of the chopper. Thus either 
+          a fermi_chopper or a disk_chopper group is required. 
+       </doc>
      </group>  
     </group>
 </definition>

base_classes/NXlog.nxdl.xml

@@ -69,9 +69,13 @@
 			and in the units specified in the "units"
 			attribute. Please note that absolute
 			timestamps under unix are relative to ``1970-01-01T:00:00``.
-        </doc>
-        <attribute name="start" type="NX_DATE_TIME" />
-	<attribute name="scaling" type="NX_NUMBER"/>
+
+			The scaling_factor, when present, has to be applied to the time values in order 
+			to arrive at the units specified in the units attribute. The scaling_factor allows 
+			for arbitrary time units such as ticks of some hardware clock.  
+		</doc>
+		<attribute name="start" type="NX_DATE_TIME" />
+		<attribute name="scaling_factor" type="NX_NUMBER" />
 	</field>
 	<field name="value" units="NX_ANY" type="NX_NUMBER">
 	  <doc>

manual/source/datarules.rst

@@ -106,6 +106,19 @@ adequately documented.
 	    Rather than persist in using names not specified in the standard, please suggest additions to the :ref:`NIAC`.
 
 
+The data stored in NeXus fields must be readback values. This means values as read from the detector, other hardware etc. 
+There are occasions where it is sensible to store the target value the variable was supposed to have. In this case the target 
+value is stored with a name built by appending _set to the normal NeXus field name.  An example:
+
+.. code-block:: text
+    :linenos:
+
+    temperature
+    temperature_set
+
+The temperature field will hold the readback from the cryostat/furnace/whatever. The field temperature_set will hold 
+the target value for the temperature as set by the experiment control software. 
+
 .. _Design-ArrayStorageOrder:
 
 NeXus Array Storage Order