Use astropy modelling for line profile fitting

[andycasey]

Including for when synthesis is required. A custom model may be required, but it would be awesome.

Remaining functionality required:

• [ ] Custom masks
• [ ] Continuum coefficients with some max_degree
• [ ] Ability to easily set fixed/free continuum?
• [ ] Switch wavelength_tolerance to a cleaner Redshift model
• [ ] Return a list of CompoundModels
• [ ] Fit just some element profiles
• [ ] Save the CompoundModels to the internal model class.

[andycasey]

The code is a bit murky right now and is in need of a refactor. Removing 'help wanted' flag while I fix it up..

[andycasey]

.