Week of April 13 - The decorators IMP::atom::AtomDecorator, IMP::atom::ChainDecorator, IMP::atom::ResidueDecorator etc. now inhertic from IMP::atom::MolecularHierarchyDecorator. This means they can set their IMP::atom::MolecularHierarchyDecorator::Type properly and saves a lot of typing in the common case. We can always make hierarchy decorators zero memory overhead if we find that the cost matters. Unless anyone objects, AtomDecorator will soon cease to inherit from XYZDecorator.

- The base classes IMP::Comparable, IMP::NullDefault, IMP::ValidDefault and IMP::UninitializedDefault were added to document what the default constructors and comparisons do for various objects in IMP.