[Nexus] NeXus definitions version: need ability to change this

[Pete Jemian]

Regarding the version number of the NeXus definitions (3.1.0, tagged on 
2016-05-29, 866 commits since that), an issue has been prepared to find 
a way for the NIAC to update the version number of the NeXus definitions 
at their choosing.

https://github.com/nexusformat/definitions/issues/387#issuecomment-219328772

Please direct ALL discussion to the GitHub issue.

At today's telco, it was agreed that this should be a topic to be 
addressed at the next NIAC in October.  We also agreed that what is 
discussed below is one aspect of the overall topic of versioning for 
NeXus definitions.

There are four known uses of the NeXus definitions version:
(1) version of each NXDL file
(2) version of the suite of NXDL definitions (*.nxdl.xml)
(3) version of the rules for writing NXDL files (nxdl.xsd and related)
(4) version used in the XML namespace

Here is that proposition:

Each NXDL file has a "version" attribute.  This solves (1).

Item (2) may not be necessary to solve.  Discussion?

Changes in item (3) are the best motivation for a new version number. 
One example for an update to the version number might due to be addition 
of the NIAC 2014 method to identify the default data to be plotted.

Item (4): Leave the XML Namespace string unchanged.  It is:
   http://definition.nexusformat.org/nxdl/3.1

Again, comments go to the GitHub issue:
https://github.com/nexusformat/definitions/issues/387#issuecomment-219328772

Regards,
    Pete