kiji-modeling: EXP-311. Rename Cell to FlowCell.

issue: https://jira.kiji.org/browse/EXP-311
review: https://review.kiji.org/r/1393
commit: https://github.com/kijiproject/kiji-modeling/commit/e8f60f6b44a491b8255c0bc40f8db391951058d9