Changed 'mandatory' to 'recommended'

It is my understanding that we agreed that the requirement for at least one NXdata group was not mandatory, but strongly encouraged.
nexusformat · rayosborn · Nov 27, 2017 · Nov 27, 2017 · Nov 27, 2017 · Nov 27, 2017
commit 0dec12d7f4ae7a03bdda57ddf635389f0ea3742a
diff --git a/base_classes/NXdata.nxdl.xml b/base_classes/NXdata.nxdl.xml
@@ -140,7 +140,7 @@
 
 		.. index:: plotting
 
-		It is mandatory  that there is at least one :ref:`NXdata` group 
+		It is recommended that there is at least one :ref:`NXdata` group 
 		in each :ref:`NXentry` group.  
 		Note that the ``variable`` and ``data`` 
 		can be defined with different names.