Handle calibration constant variants

This is wildly overdue because I'm lazy (been in pycalibration for a while. AFAIK, this is only relevant for AGIPD SlopesPC: at some point, the notebook started sanitizing the fit parameter values so we don't have to every time we load them. To know if this was done, we should look for the variant attribute of the data group. In online, we can hopefully assume we'll only get fresh constants where the variant is at least 1, but it's better to have it and check (could become important for other constants, too).