Fix WCS unit handling

After discussion with @mhvk, I changed the default behaviour slightly that a unit is not needed in initializing a WCS (if that is the case - the user is warned that this is not recommended). On the `.wavelength`, `.frequency`, ... side, I have now changed it that attributes that the current wcs system can not convert to are thrown out. So if the wcs unit=`None` then `spec1d.wavelength` will raise an attribute error. This should make it easier to add all sorts of `.xxx` attributes to `Spectrum1D`. 

This PR is based on astropy/specutils#45 so should be merged after.