Rewritten SingleElementCmdPath.compare to use sub operator from
CmdPathElem
Rewritten OrderedCmdPath.compare to use sub operator from CmdPathElem
Added CmdPathElem.__bool__
Added CmdPathElem.isunique method.
Removen unnecesary CmdPathElem.__iter__()
Added more str(CmdPathElem) tests. Just to be sure that data is not
accessed through CmdPathElem.__iter__()