p-value computation: Saddlepoint approximation when davies does not converge

The davies-method now falls back to the saddlepoint approximation as suggested in Wu et al. (2016) [1].

1.Wu, B., Guan, W. & Pankow, J. S. On efficient and accurate calculation of significance p-values for sequence kernel association testing of variant set. Ann Hum Genet 80, 123–135 (2016).