[HS3] Bugfixes to HS3 histfactoy_dist handling

[Phmonski]

This Pull request:

Patch to HS3 histfacory import and export:

• added a default error for normsys modifier
• changes the calculation of data.errors.
• the generic RooGaussian streamer function exported named RooConstVars as RooRealVars with isConstant(True). Changed the behavior to export RooConstVars as literals.

Changes or fixes:

• Histfactory normsys modifier get a default value of 0.0
• Previously, Histfactory used "constrain_p->getX()" to calculate the `erel for export. This resulted in issues when global observables changed in between loading and exporting the Workspace.
• Added a RooGaussianStreamer class that discriminates between RooRealVars and RooConstVars at export.

Checklist:

• tested changes locally

@cburgard @will-cern