dbeta_dr_ell: Same as 'dbeta_dh', but with same inputs as 'dbeta_ds' and...
In dcgerard/updogAlpha: Using Parental Data for Offspring Genotyping

Same as dbeta_dh, but with same inputs as dbeta_ds and dbeta_dl.

1	dbeta_dr_ell(x, n, d, ell, p, r)

`x`	The number of counts of reference allele.
`n`	The number of counts of reads.
`d`	The sequencing bias parameter.
`ell`	Expit of eps. We have eps = exp(ell) / (1 + exp(ell)), or ell = log(eps / (1 - eps))
`p`	The proportion of genome that is the reference allele.
`r`	We have `tau = 1 / (1 + exp(r))`