Correct and test slicing of mesocrystals with >1 different constituent particles
There are considerable complications in the code (slice2admixtures) for handling mesocrystals that are made of more than one type of particles. None of this is covered by tests. Preliminary results indicate that the computation does not work correctly (commit e8456435 in branch i335c: test Mesocrystal3 gets error from checkVolumeFractions).