Added SurfaceMGXS and Current classes to mgxs.py
Added SurfaceMGXS and Current classes to mgxs.py

Sam Shaner wrote classes to include current in the mgxs file.
Since that code was never merged, it fell severely out of date with the OpenMC develop branch.
I have restored compatibility of the code and tested the current mgxs class.
I would like someone to confirm whether _divide_by_density should be True for the SurfaceMGXS class.

walshjon/openmc,paulromano/openmc,amandalund/openmc,mit-crpg/openmc,walshjon/openmc,liangjg/openmc,amandalund/openmc,mit-crpg/openmc,walshjon/openmc,mit-crpg/openmc,shikhar413/openmc,liangjg/openmc,amandalund/openmc,paulromano/openmc,mit-crpg/openmc,walshjon/openmc,amandalund/openmc,shikhar413/openmc,paulromano/openmc,liangjg/openmc,liangjg/openmc,shikhar413/openmc,shikhar413/openmc,paulromano/openmc