Allow dimensions in NXdata to be lists of strings (#1246)

* Add new datatype, the union of NX_CHAR and NX_NUMBER
* Change AXISNAME field of NXdata to allow character values
* Add NXdata attribute default_slice: which slice of data to show in a plot by default. This is useful especially for datasets with more than 2 dimensions.

Co-authored-by: Sophie Hotz <sophie.hotz@dectris.com>

base_classes/NXdata.nxdl.xml

@@ -264,6 +264,51 @@
  https://www.nexusformat.org/NIAC2018Minutes.html
  </doc>
  </attribute>
+ <attribute name="default_slice" type="NX_CHAR_OR_NUMBER">
+ <doc>
+ Which slice of data to show in a plot by default. This is useful especially for
+ datasets with more than 2 dimensions.
+
+ Should be an array of length equal to the number of dimensions
+ in the data, with the following possible values:
+
+ * ".": All the data in this dimension should be included
+ * Integer: Only this slice should be used.
+ * String: Only this slice should be used. Use if ``AXISNAME`` is a string
+ array.
+
+ Example::
+
+ data:NXdata
+ @signal = "data"
+ @axes = ["image_id", "channel", ".", "."]
+ @image_id_indices = 0
+ @channel_indices = 1
+ @default_slice = [".", "difference", ".", "."]
+ image_id = [1, ..., nP]
+ channel = ["threshold_1", "threshold_2", "difference"]
+ data = uint[nP, nC, i, j]
+
+ Here, a data array with four dimensions, including the number of images
+ (nP) and number of channels (nC), specifies more dimensions than can be
+ visualized with a 2D image viewer for a given image. Therefore the
+ default_slice attribute specifies that the "difference" channel should be
+ shown by default.
+
+ Alternate version using an integer would look like this (note 2 is a string)::
+
+ data:NXdata
+ @signal = "data"
+ @axes = ["image_id", "channel", ".", "."]
+ @image_id_indices = 0
+ @channel_indices = 1
+ @default_slice = [".", "2", ".", "."]
+ image_id = [1, ..., nP]
+ channel = ["threshold_1", "threshold_2", "difference"]
+ data = uint[nP, nC, i, j]
+
+ </doc>
+ </attribute>
  <attribute name="AXISNAME_indices" type="NX_INT">
  <!-- 
  nxdl.xsd rules do not allow us to show this as a variable name 
@@ -294,14 +339,17 @@
  </attribute>
 
  <!-- Data and coordinate fields -->
- <field name="AXISNAME" type="NX_NUMBER" nameType="any">
+ <field name="AXISNAME" type="NX_CHAR_OR_NUMBER" nameType="any">
  <doc>
  Coordinate values along one or more :ref:`data &lt;/NXdata/DATA-field&gt;` dimensions. The rank must be equal
  to the number of dimensions it spans.
 
  As the upper case ``AXISNAME`` indicates, the names of the ``AXISNAME`` fields can be chosen :ref:`freely &lt;validItemName&gt;`.
  The :ref:`axes &lt;/NXdata@axes-attribute&gt;` attribute can be used to find all datasets in the
  ``NXdata`` that contain coordinate values.
+
+ Most AXISNAME fields will be sequences of numbers but if an axis is better represented using names, such as channel names,
+ an array of NX_CHAR can be provided.
  </doc>
  <attribute name="long_name"><doc>Axis label</doc></attribute>
  <attribute name="units">

nxdlTypes.xsd

@@ -466,6 +466,7 @@
  nxdl:NX_POSINT
  nxdl:NX_QUATERNION
  nxdl:NX_UINT
+ nxdl:NX_CHAR_OR_NUMBER
  "/>
  </xs:simpleType>
 
@@ -619,4 +620,14 @@
  <xs:restriction base="xs:unsignedByte" />
  </xs:simpleType>
 
+ <xs:simpleType name="NX_CHAR_OR_NUMBER">
+ <xs:annotation>
+ <xs:documentation>Any valid character string or NeXus number representation</xs:documentation>
+ </xs:annotation>
+ <xs:union memberTypes="
+ nxdl:NX_NUMBER
+ nxdl:NX_CHAR
+ " />
+ </xs:simpleType>
+
 </xs:schema>