Refs #9719.  Added usage example of FitPowderDiffPeaks.

On branch feature/9719_lebail_usage_example

      modified:   ../Mantid/docs/source/algorithms/FitPowderDiffPeaks-v1.rst
      new file:   ../../Test/AutoTestData/UsageData/2011B_HR60b3.irf
      new file:   ../../Test/AutoTestData/UsageData/PG3_11487-3.dat