add some safety notes to vaporcalc POD

	modified:   lib/App/vaporcalc.pm
PG/VG cmd subject outlines, improved Cmd role

	modified:   lib/App/vaporcalc/Cmd/Subject/PG.pm
	new file:   lib/App/vaporcalc/Cmd/Subject/VG.pm
	modified:   lib/App/vaporcalc/Role/UI/Cmd.pm