Using isotope ratio from SRM for calibration

[jiaor]

For radiometric dating, we need to use the isotope ratio from a reference material for calibration while we don't have good estimates on or don't need their absolute concentration. Instead of using the concentration of each isotope, please add a feature to allow using ratios, e.g., Pb206/U238, Pb207/U235, Pb207/Pb206, etc. directly to construct the calibration slope.