Replace 'dataset' with 'field'

Data sets can mean a multitude of things in different contexts. For example, in many data catalogs, data sets refer to collections of data objects. Even within this file, some might construe 'dataset' to refer to the plottable data. However, there are many uses of the word 'field', which is less ambiguous, so I would propose that this is used wherever we are referring to an item in the group.

base_classes/NXdata.nxdl.xml

@@ -42,9 +42,9 @@
 	-->
 
 	<symbols>
-		<doc>These symbols will be used below to coordinate datasets with the same shape.</doc>
 		<symbol name="dataRank"><doc>rank of the ``data`` field</doc></symbol>
 		<symbol name="n"><doc>length of the ``variable`` field</doc></symbol>
+		<doc>These symbols will be used below to coordinate fields with the same shape.</doc>
 		<symbol name="nx"><doc>length of the ``x`` field</doc></symbol>
 		<symbol name="ny"><doc>length of the ``y`` field</doc></symbol>
 		<symbol name="nz"><doc>length of the ``z`` field</doc></symbol>
@@ -54,13 +54,13 @@
 		<doc>
 			.. index:: plotting
 
-			Declares which dataset is the default. 
-			The value is the name of the dataset to be plotted.
-			A field of this name *must* exist (either as dataset 
-			or as a link to a dataset).
+			Declares which field contains the default plottable data. 
+			The value is the name of the field to be plotted.
+			A field of this name *must* exist (either as field 
+			or as a link to a field in another group).
 
 			It is recommended (as of NIAC2014) to use this attribute
-			rather than adding a signal attribute to the dataset.
+			rather than adding a signal attribute to the field.
 			See http://wiki.nexusformat.org/2014_How_to_find_default_data
 			for a summary of the discussion.
 		</doc>
@@ -163,10 +163,10 @@
 		to provide a default plot for the data of this :ref:`NXentry`.  The actual data
 		might be stored in another group and (hard) linked to the :ref:`NXdata` group.
 
-		* Each :ref:`NXdata` group will define only one data set
-		  containing plottable data, dimension scales, and 
-		  possibly associated standard deviations.
-		  Other data sets may be present in the group.
+		* Each :ref:`NXdata` group will define only one field
+		  containing plottable data, one for each of the dimension scales, and, 
+		  optionally, associated uncertainties. Other fields may be present in 
+		  the group.
 		* The plottable data may be of arbitrary rank up to a maximum
 		  of ``NX_MAXRANK=32``.
 		* The plottable data will be named as the value of 
@@ -180,7 +180,7 @@
 		      mr: float[100]  --> the default independent data
 
 		  The field named in the ``signal`` attribute **must** exist, either
-		  directly as a dataset or defined through a link.
+		  directly as a field or defined through a link.
 
 		* The group ``axes`` attribute will name the
 		  *dimension scale* associated with the plottable data.
@@ -200,7 +200,7 @@
 		The preferred method to associate each data dimension with
 		its respective dimension scale is to specify the field name
 		of each dimension scale in the group ``axes`` attribute as a string list.
-		Here is an example for a 2-D data set *data* plotted 
+		Here is an example for 2-D data, *data*, plotted 
 		against *time*, and *pressure*.  (An additional *temperature* data set 
 		is provided and could be selected as an alternate for the *pressure* axis.)::