added spatial source filter by fissionable material
added documentation for fission box source
Merge branch 'fission_src' of github.com:bhermanmit/openmc into fission_src
clear particle after fission sampling
moved location where materials are set to fissionable
default fissionable material to false
accounted for void material
fixed variable names when setting materials to fissionable
if in void material resample particle site and moved particle initialize for initial source in fission regions
must cycle when encountering material void when performing initial fission source box
Merge pull request #312 from bhermanmit/fission_src

Added capability to filter box source by fissionable material