fix a bug: if you create a new spectral cube with stringy units ('km/s'), it will fail on writing because other functions assume _spectral_unit is a u.Unit
fix a bug: if you create a new spectral cube with stringy units
('km/s'), it will fail on writing because other functions assume
_spectral_unit is a u.Unit

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