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.
I would suggest that this subroutine be in-lined in ace.F90:read_xs, e.g. see [this](https://github.com/paulromano/openmc/commit/e6a4c51020e069c9b2bf03367d8126f395b30eef).