QC for GammaPos_Z21 (HomSap)

[petrelharp]

PR for new model: #1656
Original paper: https://genome.cshlp.org/content/31/1/110.abstract
Parameter values: see discussion in #1469

Potential issues that might lead to differences between the model implementations:
none; note that this has essentially been QC'ed as I independently got the parameter estimates

QC'er requests:

[petrelharp]

Closed in #1694