find_rmax_vir

cosmic.sample.cmc.elson.find_rmax_vir(r_max, gamma)

This is a little tricky: because the virial radius of the Elson profile depends on the maximum radius, if the profile is very flat (e.g. gamma~2) then you need a large maximum radius to get a large number of virial radius in there. This basically finds r such that r / rVir(r) = r_max. In other word, how far out do we need to go to have r_max number of virial radii in the sample.