#8 Add IntCal20 calibration curve

The new IntCal20 calibration curve has been published, or is being published shortly and covers a wider timespan with increased detail: https://doi.org/10.1017/RDC.2020.22 The methodology to obtain the has been updated so, according to the authors: > the construction methodology provides not just a single potential 14C history but rather a large set of possible histories, each of which we call a realization. The published IntCal20 curve is a summary of all of these individual plausible histories— the posterior mean and variance of a large collection of realizations at each calendar age independently. These curve summaries provide the correct calibrated age for a single individual determination in a fast and efficient manner I couldn't find the published IntCal20 curve however.