GroupManager: use GroupId instead of string for version lookup
Tests: import postFix feature, to vanish compiler warning
GroupResource: Put + Post can be used interchangeably, since changing creating in a tree is interchangeably
GroupResource: Delete if not root path, is an update as well
Group: create, update, delete, get in a hierarchical fashion
GroupManager: if a group does not exist, create it, if a path does not exist create it
GroupTest: add tests for all group functions
GroupResourceTest: first resource tests