Schema update 555

[prjemian]

• changes to XML Schema, backwards-compatible
• NXtransformations has invalid unit: AXISUNITS #545 NXtransformations needs new units category: NX_TRANSFORMATION
• NXmx uses minOccurs where it is not allowed #546 NXmx use of minOccurs changed to optional
• NXtransformations adds units attribute to vector attribute #547 NXtransformations correction
• NXmx has incorrect use of "dimensions" #556 allow declarations of dimensions for data values in attributes

locally: utils/test_suite.py passes and the Sphinx documentation builds

[prjemian]

We should bump the minor version number of the XML Schema. Here is the definition from nxdl.xsd:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://definition.nexusformat.org/nxdl/3.1"
xmlns:nx="http://definition.nexusformat.org/nxdl/3.1" 
elementFormDefault="qualified">

BUT, that will change the namespace and that can have consequences that affect anything that uses the XML Schema.

Should we bump the minor version? Any other way to differentiate from the previous XML Schema?

[zjttoefs]

Reading the diff it all looks sensible to me. Since the tests pass this also appears to work. Good enough for me.

[prjemian]

@yayahjb Comments? tacit approval is not satisfying

[prjemian]

I will examine the consequences of changing the minor version number without changing the namespace string. It may well be that we leave the namespace string as-is, at least for now.

[zjttoefs]

Sorry, forgot to comment on the version number. I would say: Bump it.
We might get away not changing it, but having version numbers we're too afraid to ever change is a bit pointless. We will see what breaks and have to prepare that for future updates.

[yayahjb]

Looks good. -- HJB