added cross-correlation routine
added cross-correlation routine

low-sky/pyspeckit,bsipocz/pyspeckit,e-koch/pyspeckit,bsipocz/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit,jpinedaf/pyspeckit,keflavich/pyspeckit-obsolete,allisony/pyspeckit,vlas-sokolov/pyspeckit,e-koch/pyspeckit,vlas-sokolov/pyspeckit,dinossimpson/pyspeckit,pyspeckit/pyspeckit,pyspeckit/pyspeckit,keflavich/pyspeckit,keflavich/pyspeckit,mikelum/pyspeckit,low-sky/pyspeckit,mikelum/pyspeckit,dinossimpson/pyspeckit