Use decorator @unittest.skipIf to skip unit tests in parallel for
adaptivity, ale, book, fem. Need to sort out the rest.
Replace all ad hoc checks for skipping specific unit tests in parallel
with Python >= 2.7 decorator:

@unittest.skipIf(MPI.size(mpi_comm_world()) > 1, "Skipping unit test(s) not working in parallel")
More unit test fixes
More unit test fixing
Merge branch 'logg/fix-issue-42' into next