change .flux to quantity

This change has been suggested both by @followthesheep and @mhvk as a current inconsistency.

While `spec1d.wavelength` gives back a `Quantity` - `spec1d.flux` only returns an `ndarray`. This PR fixes this and makes `spec1d.flux` a quantity. 

@nhmc @keflavich you guys might use this. Does that make sense to you?