added >0 masking to powerlaw fit for baseline
added >0 masking to powerlaw fit for baseline

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