Reader iraf strict

Improving the FITS reader, now it conforms strictly to IRAF format. Added an IRAF combination class to hold multiple WCS as well as metadata for multispec format, and changed the fits reader to take in multiple WCS per spectra.
@wkerzendorf @nhmc 

@embray This is essentially a `SerialCompositeModel` can you help @shaileshahuja to make sure that we stick with your new modelling structure and use this functionality there.