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.
If the call the `find_cell` results in a void cell, found will be .true. and the do loop will exit.