First work on ve center alternative to discretize_geom
Added cell at ve center alternative to discretize_geom.
Merge branch 'staging' of github.com:pyne/pyne into ve_center
discretize_geom now handles unstructured mesh.
Made num_rays and grid into kwargs.
Merge branch 'staging' of github.com:pyne/pyne into ve_center
Docstring fix.
Public API for ray tracing discretization.
Added stuctured check to new public API.
Merge branch 've_center' of https://github.com/elliottbiondo/pyne into elliottbiondo-ve_center