Replaces The GroupTree + GroupTreeNode classes building an explicit tree
of named nodes with a sorted vector of { name, parent-name } pairs that
builds an implicit tree. Provides the same semantics as the previous
group tree implementation, but with less code and copying for free.
Also makes the *contents* of the vector values, not shared_ptrs.
This introduces the copy constructor (as an alternative to deepcopy())
and comparison operators, mandated by DynamicState.
