bugfix - in cubes, need to check for nans before computing stddev
bugfix - in cubes, need to check for nans before computing stddev

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