AKq02a0 Andreev Kingkade a0 method. This version has a 3-part segmented linear model, based on cut points in q0. Code ported from HMDLifeTables.
a value or vector of values of q0, the death probability in the first year of life.
either "m" or "f"
a0, the estimated average age at death of those dying in the first year of life, either a single value or a vector of a_0 values.