dbeta_dl: Derivative of beta density w.r.t. unconstrained...
In dcgerard/updogAlpha: Using Parental Data for Offspring Genotyping

I use the chain rule here. dbeta_dprop * dxi_df * df_deps * deps_dell.

1	dbeta_dl(x, n, d, ell, p, h)

`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.
`h`	the overdisperion parameter. tau = 1 / (h + 1) or h = (1 - tau) / tau