This function wraps the two approximations for a0 based on either q0 (IMR) or m0.
lt_rule_1a0_ak(M0 = NULL, q0 = NULL, Sex)
a value or vector of values of `1m0``, the death risk in the first year of life.
a value or vector of values of `1q0``, the death probability in the first year of life, sometimes approximated with IMR.
a0, the estimated average age at death of those dying in the first year of life, either a single value or a vector of values.