Modifed generate_x function
Merge branch 'dev-kun' of https://github.com/kunyuan/DiagMC-time into dev-kun
change generate_tau and prob_dtau
Revert "change generate_tau and prob_dtau"

This reverts commit 9f388ea53a7ec4e2f8b3e7d9e87afd12ccb9d405.

Oh yeah, Kun did it!
tau is uniform
Try to add log X distribution
x distribution done.
Merge branch 'dev-kun'
merge R distribution function