General test for checking whether nodes are in trees for non-kinetic groups
Mark the StatMech group/tree test as Work In Progress

Per the discussion at https://github.com/GreenGroup/RMG-Py/pull/242
it's not clear if this test failure is really a problem or not,
or how best to resolve it. For now I'm marking as WIP so that 
we can get the rest of the tests in Master without Travis failing.
Minor tidy-up of databaseTest.py
Merge branch 'pr/242' to get more database tests in databaseTest.py