Cep18

This is a companion to cyclus/cyclus#641 and cyculs/cyclus#644. It is nominally complete!

Specifically the tasks include:

Implementation:
- [x] SourceFacility
- [x] SinkFacility
- [x] EnrichmentFacility
- [x] BatchReactor

Testing:
- [x] SourceFacility
- [x] SinkFacility
- [x] EnrichmentFacility
- [x] BatchReactor

Input files:
- [x] update

Edit: 
As of 17d601f, all the implementation code has been finished. The only things left (which I'm getting to now) are logging, some documentation, and minimal testing of the BatchReactor. @rwcarlsen and @scopatz, this is ready for review. However note that it requires cyclus/cyclus#653.

Finally, @scopatz, I overhauled the BatchReactor to have a more intuitive and basic implementation. I believe that in its current form, it should be suitable for both the reactor and reprocessing facility in the proposed simulation. I invite you to take a look (perhaps after I add some more thorough documentation, so you don't have to dig as much).