Seems to work so far Hint Level 1
Fixed some layout issues + added unique/not unique status message
Remove uniques now seems to work, but need to rejig to make useful.
Massive performance improvement by sorting cells by # markups left
import cleanup
removed comments
Simplified version of Puzzle::empty() using Java 8 construct
Cleanup - most methods in Nonet can be in AbstractUnit
Changed removeLimit - still not fast enough
Revamp of Cell, carrying its own limit.
Added test case for hints