fixed bug preventing mix Ar/Xe geometry (TRT_Digitization-01-00-19)

2015-02-11  Andrew Beddall  <Andrew.Beddall@cern.ch>

        * src/TRTDigitizationTool.cxx
          from working. Now fixed, but warnings are not
          given if TRT/Cond/StatusHT is not set up correctly!

        * Tagged as TRT_Digitization-01-00-19

2015-02-10  Andrew Beddall  <Andrew.Beddall@cern.ch>

        * Configured to expect use of TRT/Cond/StatusHT by default
          for setting straw gas in mixed Xe/Ar geometry; that is
          initialized m_useConditionsHTStatus as "true".
          If the folder is absent a warning is given and it
          *defaults back to full Xenon*.

        * Changed the behaviour of IsArgonStraw() so that m_UseArgonStraws
          (set with a postExec) will set all straws to Argon, but is
          *overridden* by TRT/Cond/StatusHT if available.

...
(Long ChangeLog diff - truncated)