Revert "Use mt19937 instead of rand for number generation."
Revert "Use mt19937 instead of rand for number generation."

This reverts commit 2d483650baf465fb1065af915907f1094f0be840.

I want to see if the performance change in the last build was related to the change in random number generation, or an indication of a bug in the ising model.

marblar/demon,marblar/demon,marblar/demon,marblar/demon