TrkVertexFitterInterfaces, etc: Make IVertexFitter const.
Make the fit() methods of IVertexFitter const. Needed to fix ToolHandle constness issues.
Also update TrkVKalVrtFitter, TrkVertexBilloirTools, TrkVertexFitters, TrkV0Fitter.
Make the fit() methods of IVertexFitter const. Needed to fix ToolHandle constness issues.
Also update TrkVKalVrtFitter, TrkVertexBilloirTools, TrkVertexFitters, TrkV0Fitter.