when evaluating  the interpolation integral with a > b
  Return now always the - Integral(b,a)

- Check now error returns when calling the evaluation of th einterpolation function and report 
 eventually the error returned by the GSL function  
(this can happen when evaluating outside the domain)


git-svn-id: http://root.cern.ch/svn/root/trunk@31965 27541ba8-7e3a-0410-8455-c3a389f83636