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?
We should not have a `MultiSpecSpectrum1D`. This is mixing the serialization options with the logical constructs of a spectrum, meaning MultiSpec is a invention of IRAF of saving multiple spectra in a file. We should have an API that can read/write from these fits files, but should not have a class that caters to them.