cleaned up the constructor of KItem; cached LIST_TERMINATOR
minor refactoring
cached results of getMaximalLowerBounds method; fixed a latent bug in getGLB when there are multiple LBs but no GLB
forgot to cache different list terminators with different separators
added unit tests for Poset; also fixed getLUB in order to pass the unit tests
Merge pull request #601 from yilongli/cleanup-KItem-constructor

Cleanup k item constructor