compute genomic inflation factor

[timflutre]

Known as lambda:

median(z^2) / 0.455 where qchisq(0.5, 1) = 0.455

The GIANT Consortium, Yang, J., Weedon, M.N., Purcell, S., Lettre, G., Estrada, K., Willer, C.J., Smith, A.V., Ingelsson, E., O’Connell, J.R., et al. (2011). Genomic inflation factors under polygenic inheritance. European Journal of Human Genetics 19, 807–812.

Take p values as inputs and return lambda.
Include it in the function which runs GEMMA uLMM.