allow an equivalency to be specified for Jy<->K in LDOs, don't force them to have frequency defined
allow an equivalency to be specified for Jy<->K in LDOs, don't force
them to have frequency defined

keflavich/spectral-cube,low-sky/spectral-cube,e-koch/spectral-cube,radio-astro-tools/spectral-cube,jzuhone/spectral-cube