NameTree::eraseEntryIfEmpty is renamed to eraseIfEmpty;
the deprecated overload that accepts shared_ptr<name_tree::Entry> is deleted.
name_tree::Entry::getPrefix is deleted in favor of getName.
This commit also improves Doxygen in NameTree class.
refs #3687
Change-Id: Ia98ca676ce6d3bc7b2e97328adccac911c8167d7