[Bug-apl] RNG

[Xiao-Yong Jin]

Hi,

The LCG used for roll may be fine for some casual uses, but I would really like 
to see a higher quality RNG adopted here.  Since speed may not be the main 
concern here, employing the use of c++11 <random> and preferably using the 
std::mt19937_64 seems to be much better for any monte carlo type calculations.  
It could be a trivial change to Quad_RL class, although saving the RNG state in 
a workspace may require a bit more code.  What do you say?

By the way, since in Workspace::get_RL 'return rand % mod;', you can remove the 
same ‘%’ in all the bif_roll definitions.

Best,
Xiao-Yong