Add implied defaults for sclaing_factor and offset

applications/NXmx.nxdl.xml

@@ -171,12 +171,16 @@
  single per-pixel array applied to every image (dimensions (i, j) or (i, j, k)),
  or a per-image correction specified with an array whose slowest rank is nP (dimensions
  (np, 1), (np, i, j) or (np, i, j, k)).
+
+ When ommited, the scaling factor is assumed to be 1.
  </doc>
  </field>
  <field name="data_offset" type="NX_NUMBER" optional="true">
  <doc>
  An optional offset to apply to the values in data.
 
+ When ommited, the offset is assumed to be 0.
+
  See :ref:`data_scaling_factor &lt;/NXmx/ENTRY/DATA/data_scaling_factor-field&gt;` for more information.
  </doc>
  </field>

base_classes/NXdata.nxdl.xml

@@ -464,12 +464,16 @@
  corrected values = (FIELDNAME + offset) * scaling_factor
 
  This formula will derive the values to use in downstream applications, when necessary.
+
+ When ommited, the scaling factor is assumed to be 1.
  </doc>
  </field>
  <field name="FIELDNAME_offset" type="NX_NUMBER">
  <doc>
  An optional offset to apply to the values in FIELDNAME (usually the signal).
 
+ When ommited, the offset is assumed to be 0.
+
  See :ref:`FIELDNAME_scaling_factor &lt;/NXdata/FIELDNAME_scaling_factor-field&gt;` for more information.
  </doc>
  </field>