Multispec linear fits

Don't merge until https://github.com/astropy/specutils/pull/74 is merged. 
I have added support for reading and writing linear and log-linear multispec format. Also added Doppler WCS and Log WCS as separate entities. They do complicate the code a bit, but it gives the user more flexibility. I dont know how important it is, @wkerzendorf @nhmc your thoughts?
@embray this is in itself a `SerialCompositeModel` calling multiple models sequentially. Have you had a chance to restructure the Composite models? Should we inherit from them. @shaileshahuja I think will have some more questions.