Calculates the variance of a standardized mean difference using the method of Hedges and Olkin (1985):
compute_smd_var(d, smd_parts)an SMD value
a list of components for from compute_smd_parts
computing standardized mean differences
$$ \sqrt{\frac{n_1 + n_2}{n_1n_2} + \frac{d^2}{2(n_1 + n_2)}} $$