Parallelization of SLiM with OpenMP

[bhaller]

This PR is a place for discussion and review of changes on the parallel branch. This is experimental code, and may contain serious bugs. End users should not use it. At the moment I am assuming that I am the only person using this branch, so I WILL FORCE-PUSH TO THIS BRANCH WITHOUT WARNING! If you intend to start using this branch, please let me know!