[Rivet] errors scaling in DPS

[Anton Karneyeu]

Hi Andy, hi all,

looks like there is an error in negative errors scaling in DataPointSet 
- see calls of setErrorMinus() in:

 
http://projects.hepforge.org/rivet/trac/browser/trunk/include/LWH/DataPointSet.h?rev=3186

Could you take a look?

Cheers,
Anton