Add version to Nexus files
As requested by @payno we need to version the Nexus files to deal with changes.
Should we separate Nexus writer from Bliss version @matias.guijarro?
Currently the "creator" attribute of the NXroot is simply "Bliss". Shall I add the version number here @sole? The nexus standard does not seem to foresee an "@creator_version" attribute.
Edited by Wout De Nolf