Fixed issue #8182: (quexf-pdf) labels for dual matrix question

Dev: advanced attributes dualscale_headerA and
Dev: dualscale_headerB are now used in the
Dev: quexf-PDF to label the response axes.

commit
Merge pull request #129 from mfaber/quexf_dualmatrixlabel205

Fixed issue #8182: (quexf-pdf) labels for dual matrix question