Cep18

This is a companion to cyclus/cyclus#641 and cyculs/cyclus#644. It is *not* complete.


Specifically the tasks include:

Implementation:
- [x] SourceFacility
- [x] SinkFacility
- [ ] EnrichmentFacility
- [ ] BatchReactor

Testing:
- [x] SourceFacility
- [ ] SinkFacility
- [ ] EnrichmentFacility
- [ ] BatchReactor