NXtransformations has invalid unit: AXISUNITS

[prjemian]

The travis-ci build server reports an error with NXtransformations

NXDL_Invalid: base_classes/NXtransformations.nxdl.xml : Element '{http://definition.nexusformat.org/nxdl/3.1}field', attribute 'units': [facet 'enumeration'] The value 'AXISUNITS' is not an element of the set {'NX_ANGLE', 'NX_ANY', 'NX_AREA', 'NX_CROSS_SECTION', 'NX_CHARGE', 'NX_CURRENT', 'NX_DIMENSIONLESS', 'NX_EMITTANCE', 'NX_ENERGY', 'NX_FLUX', 'NX_FREQUENCY', 'NX_LENGTH', 'NX_MASS', 'NX_MASS_DENSITY', 'NX_MOLECULAR_WEIGHT', 'NX_PER_AREA', 'NX_PER_LENGTH', 'NX_PERIOD', 'NX_POWER', 'NX_PRESSURE', 'NX_PULSES', 'NX_SCATTERING_LENGTH_DENSITY', 'NX_SOLID_ANGLE', 'NX_TEMPERATURE', 'NX_TIME', 'NX_TIME_OF_FLIGHT', 'NX_VOLTAGE', 'NX_VOLUME', 'NX_UNITLESS', 'NX_WAVELENGTH', 'NX_WAVENUMBER'}., line 104

A comment in the NXDL file says:

nxdl.xsd rules do not allow us to show this as a variable name
- we'll use AXISNAME in ALL CAPS for consistency with the
use in NXDATA.  We'll use ``AXISUNITS`` for the units appropriate
to the type of axis.

NXDL files must adhere to the rules of nxdl.xsd. As stated, there is no unit type that satisfies the intent in this NXDL class.

[prjemian]

One way to resolve this is to add a unit type to nxdlTypes.xsd in the simpleType anyUnitsAttr that describes this particular usage.

There will be one or more transformations (axes) defined by one or more fields
for each transformation.  The all-caps name ``AXISNAME`` designates the
particular axis generating a transformation (e.g. a rotation axis or a translation
axis or a general axis).   The all-caps name ``AXISUNITS`` designates the
units appropriate to the transformation_type value, i.e. ``NX_LENGTH`` for
``translation``, ``NX_ANGLE`` for ``rotation``, and ``NX_UNITLESS`` for
axes for which no transformation type is specified.

I propose adding NX_TRANSFORMATION with this description. Perhaps it may be possible to specify the three possibilities with an XML union structure: NX_LENGTH | NX_ANGLE | NX_UNITLESS

[prjemian]

Here is one example from the current code:

<field name="AXISNAME_end" units="AXISNAME" nameType="any" type="NX_NUMBER" minOccurs="0">
	<doc>
		``AXISNAME_end`` is a placeholder for a name constructed from the actual
		name of an axis to which ``_end`` has been appended.

		The values in this field are the end points of the motions that start
		at the corresponding positions given in the ``AXISNAME`` field.
	</doc>
</field>