Add DFE from Johri et al

[petrelharp]

We'd like to add a DFE that is piecewise-uniform; here's a good one (on Zambian Drosophila):
https://academic.oup.com/genetics/article/215/1/173/5930426?login=true
(See figure 6.)

[izabelcavassim]

will follow this:
https://github.com/paruljohri/BGS_Demography_DFE/blob/master/Scripts/demo_disc_5_SingExon_osg.slim