Added capability to filter box source by fissionable material

This PR allows the user to perform automatic rejection sampling on a box source to ensure that all particles start in a fissionable material region. The user specifies "fission" instead of "box" under the space part of the initial source designation. This takes care of issue #245.
You could probably just put `call p % initialize()` at the beginning of the do loop and remove this and the one above the do loop.