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.
That is what I intended, because the user wants particles to start in fission regions so they can wait as long as it takes and not reach the max resample limit.