Updates for latest OpenMC version

[DanShort12]

OpenMC have updated how sources are handled, so this source should be updated to align with that:

• Abstract base class name has changed to Source.
• Source is now managed by the CustomSourceWrapper.
• Sampling is done by a const method.
• The path to the source library and the representation of the parameters are given as strings, rather than XML elements
• It should be possible to remove pugixml as a dependency.

[makeclean]

when you pr this, im busy making a system to better find openmc, I was still using the master branch - totally forgotten that it changed - and im finding that it works better than this brnach

[DanShort12]

Thanks for the note @makeclean - I think I'll hold off on a PR for this until #16 is merged as they touch on a few common points. I suspect the master branch doesn't have some of the python packaging that has been worked on (and hopefully now fixed under #16), so the CMakeLists.txt is probably a bit simpler because of that.