MAGIC variance reduction

I added the MAGIC variance reduction technique to variancereduction.py. @makeclean, you should review the method. Tests were also added.

Theory manual addition still needed.

issue #344
We now have a particle class you can import and use like

      if tally.particle == "Neutron" 
             tally.particle = particle.mcnp(tally.particle)

You could even check to make sure that tally.particle is a valid PyNE particle type. 

     if particle.is_valid(tally.particle)