get_parent_outprop: E-step for the parents.
In dcgerard/updogAlpha: Using Parental Data for Offspring Genotyping

E-step for the parents.

1 2	get_parent_outprop(pcounts, psize, ploidy, pgeno, d, eps, tau, out_prop, out_mean, out_disp)

`pcounts`	The number of counts of the reference allele we observe in a parent.
`psize`	The total number of reads we observe in a parent.
`ploidy`	An integer. The ploidy of the species. This is assumed to be the same for each individual.
`pgeno`	The genotype of the parent.
`d`	Same as `bias_val` in `obj_parent`.
`eps`	Same as `seq_error` in `obj_parent`.
`tau`	Same as `od_param` in `obj_parent`.
`out_prop`	The proportion of points that are outliers. Defaults (quite arbitrarily) to `0.01`.
`out_mean`	The mean of beta-binomial for the outlier distribution. Defaults to `0.5`.
`out_disp`	The overdispersion parameter of the outlier distribution. Defaults to `1/3`, which corresponds to a uniform distribution for the underlying beta when `out_mean = 0.5`.