Allow changes of rest frequency

If you have a cube in velocity units and you want to change to a different rest frequency... you can do it with this brand new automobile!

Before, rest frequency could be overwritten, changing the physical meaning of the cube, which is bad.  Now, that should not be possible.

If you have a cube in wavelength or velocity, the rest frequency/wavelength can be (and always could have been) set with `with_spectral_unit`.

This relies on #98 and may be important for #95.