added a simpler interpolation method for when xarr is th einput
added a simpler interpolation method for when xarr is th einput

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