Fix major issue with histo ident

With @elebouli we found out that there was a major bug in the way the ident of histos worked in ScurveFitter (but also likely in ParameterAnalysis ?).

Here is a (temporary) fix for it. It could/should be improved at least in the sense that one should make sure not to exceed the "unsigned" limits and that using an index of the loop and not its value to set the offset would be better. I just need some time to do it...