check reference stamp file

• Check for the reference stamp file, art_download_AtlasBuildStamp in the same directory as the reference file.
  • This can pick up the file written by art.py download ( art-sw@82811a6c) to show the $AtlasBuildStamp for the reference file.
  • The contents are used as default for --referenceName and displayed in the plot legend.
  • If used, also moves the release label to the monitored legend.
• Fix except IOError → OSError in DCubeUtils.relpath()